A-Z

GLOSSARY OF WATCH TERMS

A-D

• Acrylic

A plastic used to make crystals. It's very soft, but is easy to repair and does not shatter when it breaks. The Omega Speedmaster Professional was chosen for spaceflight for this reason.

• Analog

A display of time or other data using dials and hands, as opposed to a digital readout. After the surge in digital quartz models in the 1980s, the traditional format has become popular again and most high-end watches now use it.

• Analog-digital display

A watch display that combines analogue and digital elements, usually found on Quartz watches, though there are some mechanical versions.

• Aperture

Small opening. The dials of some watches (in French: montres à guichet) have apertures in which certain indications are given (e.g. the date, the hour, etc).

• Applique

Applique or applied chapters are numerals or symbols cut out of a sheet metal and stuck or riveted to a dial.

• Assembling

Process of fitting together the components of a movement. This was formerly done entirely by hand, but the operations have now been largely automated. Nevertheless, the human element is still primordial, especially for inspection and testing.

• Asassortment

French term for the parts used for making an escapement.

• Anti-magnetic

A type of mechanical watch movement that is designed to be highly resistant to magnetic fields that could adversely affect or stop it.

• ATP

Army Trade Pattern, stamped on watches used by the British Armed Forces during WW2.

• Automatic winder

A box or cabinet with motorized rotating slots to hold watches with automatic movements. The rotation keeps the watch wound by spinning the movement's rotor, and accurate by counteracting the effects of gravity. Some winders are incorporated into special safes to hold collector automatic watches that cost six figures.

• Automatic

Also called self-winding. A mechanical watch movement that is wound by the motions of the wearer by either spinning a rotor or oscillating a weight. Failure to wear the watch regularly will cause it to stop once its power reserve is exhausted.

B

• Balance

The oscillator on a mechanical watch that regulates the movement so it runs in a series of beats or vibrations.

• Balance spring

See Hairspring.

• Balance wheel

An oscillating wheel that regulates the movement by dividing time into regular beats

• Balance cock

A bridge with a lug anchored to a stud. It holds the top pivot of the watch's balance staff.

• Barrel

Thin cylindrical box containing the mainspring of a watch. The toothed rim of the barrel drives the train.

• Bezel

A ring around the watch face made of stainless steel, ceramic, precious metal or base metal. It can be either decorative or movable. If the latter, it may be inside or outside the crystal and rotate to act as a timer, tachymeter, slide rule, or other function.

• Bi-color

See Two-tone

• Bi-directional rotating bezel

A bezel that can rotate either clockwise or anticlockwise to perform a function.

• Bracelet

A watch band made of movable links. These can be adjusted to fit the wearer and may have special clasps that allow them to expand to fit over a wetsuit or jacket.

• Bridge

A plate or bar set on a movement's main plate to act as a frame to hold a watch's inner workings. They can also be shaped or decorated in skeleton watches for aesthetic purposes.

• Broad Arrow

British Military marking, also known as a pheon. Now also used for shape of hour and minute hands

C

• Calatrava

A line of watches made by Patek Philippe which are simple with functional time only, often dispensing with date and sometimes even the second hand. It has become a generic term for a style of dress watch. See Calatrava for more information.

• Calendar

A complication that shows the date and may also show the day of the week, the month, or even the year. Some calendar watches need to be adjusted manually whenever the month is less than 31 days long. Others, called perpetual calendars, are self-adjusting over several decades or centuries.

• Caliber

See Movement.

• Cambered

An arched or curved bezel or dial.

• Case

The housing used to protect the movement. Usually made of steel, base or precious metal, it can also be made of plastic, carbon composite, ceramic, rubber, or other materials.

• Case back

The back or reverse of a watch designed to protect the movement. These are generally made of stainless steel, though they may include a crystal to show off the workings inside. Case backs may be simply snapped in place or screwed in with rubber gaskets to protect against water and dust. These often include information about the watch, like hallmarks, major specifications, or a serial number.

• Chapter ring

A ring of symbols or marks on a watch dial to indicate minutes or seconds.

• Chapters

Large marks on a chapter ring to denote hours. Quarter hours often have their own chapters.

• Chronograph

A complication that allows a watch to act as a stopwatch with a precision of down to a tenth or a hundredth of a second. Some versions allow lap timing as well for individual contestants in a race or events in a process.

• Chronometer

A precision timepiece capable of keeping time with a precision and accuracy suitable for navigation. Swiss-made watches are certified as chronometers by meeting the standards set by the Swiss Official Chronometer Control (COSC), which includes several days of rigorous testing under various pressures and temperatures.

• Clasp

Fastening mechanism for straps and bracelets. Leather, rubber, and cloth bands tend to use buckles, though bracelets and expensive leather straps use deployment straps for a tighter fit with less wear.

• Co-Axial Escapement

Invented by George Daniels and introduced to Omega models. The co-axial escapement is considered by some to be one of the most significant horological advancements since the invention of the lever escapement.

• Cockpit clock

All British military aviation watches of the WWI era tend to be known as ‘cockpit clocks’ because, despite their pocket watch design, most were installed temporarily in holders attached to cockpit instrument panels, being issued on a flight-by-flight basis, along with armament and ammunition. This system was developed because the watches required careful maintenance to be effective and, more importantly, were valuable and accountable items.

By WWII, of course, clocks were often installed permanently as part of the instrument panel and their movements tended to have more in common with clocks than watches.

• COMEX

The French company COMEX (Compagnie Maritime d’Expertises) – a pioneer in saturation diving – let Rolex equip its divers with the Rolex Submariner. From 1963 onwards, the Rolex Sea-Dweller was the company’s dive watch of choice because of its innovative case with the helium escape valve and its waterproofing of up to 610 metres – or 2,000 feet, earning the watch the name Sea-Dweller 2000.

• COSC

The Official Swiss Chronometer Testing Institute (COSC) in Switzerland

• Complication

Any function on a watch beyond displaying hours, minutes, and seconds. These can include even unseen things like an automatic winding function.

• Cosmograph

A chronometer developed and named by Rolex that has a tachymeter mounted on the bezel instead of the dial.

• Countdown timer

A complication that counts down in reverse from a preset time, like for timing an egg or a parking meter. This can include an alarm to alert the wearer when the time has elapsed, though that is usually found on quartz watches.

• Counterfeit watch

An illegal copy of a watch that is passed off by the maker as the real thing.

• Crown

A button on the side of the case that pulls or screws out, allowing the wearer to adjust the time and date or synchronize the seconds. In non-automatic watches, it is also used to wind the mainspring. Screw-in or screw-down crowns, or similar mechanisms, seal the watch to keep out water at extreme depths.

• Crystal

A transparent cover over the dial or in the case back, or it may make up the entire case. It can be made of ordinary glass, mineral glass, acrylic plastic, synthetic sapphire, or ruby. Sapphire is preferred for high-end watches because it's almost as hard as diamond, though many use plastic because it's easy to repair and cracks instead of shattering into small, potentially dangerous fragments.

• Cyclops

A small blister on a crystal that acts a magnifying lens to make the date more legible.

D

• Day/date watch

A watch with indicators or windows to display both the date and the day of the week. These require a more complicated mechanism to allow them to be adjusted quickly without jamming.

• Day/night indicator

A colored indicator on a dual or world time watch that shows if the home and second time zones are in daylight or night. It's particularly useful for watches without a 24-hour readout.

• Deployant clasp

A buckle that allows a strap or bracelet to stay in one piece, by opening and fastening using a hinged plate with adjustable extenders. The latter are particularly popular with divers or aviators, who may need to adjust the band to go over a wetsuit or jacket. Commonly referred to as ‘deployment clasps’.

• Depth alarm

An alarm on a divers' watch that sounds when the wearer exceeds a pre-set depth.

• Depth sensor/Depth meter

A device on a divers' watch that determines the wearer's depth by measuring water pressure. It shows the depth either by analog hands and a scale on the watch face or through a digital display.

• Dial

The face of the watch, which carries the hands, marks, indicators, and subdials. A dial may be plain, highly ornate, or divided into bridges to make the movement visible.

• Digital watch

A watch that shows the time and other information through a digital display instead of with hands. Digital watches are associated with quartz watches first marketed in the 1970s, which used LED and LCD displays, but mechanical digital watches date back to the 19th century and have appeared from time to time as novelty items. It was widely believed in the 1970s and 80s that digital watches would not only drive out analog ones, but analog timekeeping as well. However, recent decades have seen a resurgence of analog watches with digitals withdrawing to the cheapest or most specialized markets.

• Direct-drive

Refers to a seconds-hand that moves forwards in little jerks.

• Dive watch

Also called a diver or diver's watch. A watch with a case designed to remain water resistant to depths of at least 200 m (660 ft). It also includes a dial designed to be legible at a glance under poor light conditions, and may have an elapsed time bezel for timing decompression stops. These have been superseded by wrist dive computers, but most divers still wear a dive watch as a back-up. True dive watches should meet a specific standard for diving like ISO 6425.

• Doctor's watch

A watch that is robust, easy to keep clean and sterile, and has a special seconds hand and markings to allow a doctor to quickly measure a patient's pulse.

• Dolphin standard

A watch worn by skin and scuba divers with some water proofing.

• Dress Watch

A watch worn on special occasions, in the evening normally. See Dress Watches for more. A water-resistant watch that is suitable for swimming, snorkelling, boating and other water sports.

• Dual time

A watch that is able to display the time in two separate time zones simultaneously.

E-H

• Elapsed-time bezel

A rotating bezel for tracking elapsed time by lining up the zero mark with the minute hand, so the result can be read directly.

• Electric or electronic watch

A watch that runs on electric current instead of a mechanical movement. This generally refers to a quartz movement, but can also be used in regard to watches using tuning forks or other electric or electronic regulators.

• EOL

End Of Life. This is an indicator on quartz watches that shows that the battery is almost exhausted and needs replacing. Often seen as the second hand moving forward in jumps of 2-5 seconds.

• Equation of time

A complication that shows the difference between true solar time and mean solar time. The Equation of time complication shows by how much the two times diverge from one another on a particular day.

• Escapement

The part of the watch movement that divides the time into a series of beats or oscillations.

F

• Face

See Dial.

• Factory (works)

In the Swiss watch industry, the term manufacture is used of a factory in which watches are manufactured almost completely, as distinct from an "atelier de terminage", which is concerned only with assembling, timing, fitting the hands and casing.

• Field watch

A watch originally designed to be issued to soldiers for use on the battlefield. They are notable by their simple, rugged, and easy to service construction.

• Flyback

A hand on watches and especially chronographs that immediately jumps back to zero. On a regular watch, it can be used to conserve space on the dial by having a minute or second hand travel along an arc instead of revolving around the whole dial. When it reaches the end, it jumps back to zero in an instant. On chronographs, the flyback hand is used to record elapsed time, and restarts the instant it is reset to zero without the need to stop and start the watch again. It's particularly useful when repeatedly timing a series of events.

• Frequency

A measurement of a watch movement's speed as measured in the number of semi-oscillations or half-turns of the balance spring. These are measured in beats or vibrations per hour (bph, vph) or Hertz. The higher the frequency, the smoother and more accurate the movement – which is especially useful in chronographs that measure in hundredths of a second. A modern wrist watch usually has a frequency of 4 Hz (28,800 vph).

• Function

A general term for the things that a watch can do. The term is different than a complication because functions can include basic timekeeping.

• Fusee

An obsolete way of equalizing the power of a mainspring as a watch unwinds by using a chain wound around a spiral-grooved, truncated cone. As the watch winds down, the chain is unwrapped from the narrow top of the cone and onto the mainspring barrel. Because the mainspring has less force as it unwinds, the chain sitting in progressively wider grooves of the spiral compensates and the watch runs properly.

G

• Gasket

A rubber ring set in a watch case back, crown, or crystal to protect against water and dust. Improper replacement while servicing can ruin your whole day.

• Gear train

The series of gears of a watch movement that runs between the mainspring and the display or a complication. The arrangement controls the running of the watch.

• Geneva seal

Also called the "Poinçon de Genève." The official seal of the City and Canton of Geneva, Switzerland. In horology, it's used as a quality seal by the Watchmaking School of Geneva on watches made in the Canton that have been submitted for and passed a special inspection. This is based on 12 criteria regarding the movement's materials and finishing.

• Geneva waves

Also called "Geneva stripes" or "côtes de Genève." These are decorations consisting of stripes applied to a watch's plates, bridges, balance cocks or rotors.

• GMT

Greenwich Mean Time (GMT) is the time at the Prime or Zero Meridian that runs through the Royal Observatory at Greenwich, England and is not subject to summer time or daylight savings time. It's often confused with Coordinated Universal Time (UTC), but the latter is actually a precisely determined scientific time standard, which is constantly adjusted to remain in step with time as it is marked by an atomic clock.

• GMT watch

A watch with a second hour hand that is set to mark the time at Greenwich. In conjunction with a special 24-hour bezel, it allows the wearer to quickly read the time in different zones. Alternatively, the additional hand can be set to any desired second time zone.

• Gold plated

A way of giving a watch case or bracelet the look of gold by electroplating it with a layer of the precious metal. Since the thickness of such plating is only a few microns, the finish is subject to wear in daily use. For this reason, many watches are available in two-tone finishes to preserve the gold by restricting it to less vulnerable areas of the watch.

• Gold capped

• Gold rolled

• Grail Watch

A highly valued watch – usually one that's out of the price range of the person who desires it. See Grail Watch for a more detailed description!

• Grande complications

A mechanical watch that includes a large number of functions or complications. Such watches are not meant for practical use, but as showcases for craftsmanship.

• Grande sonnerie

A watch or other timepiece that chimes at the hour, half hour, and quarter-hours automatically or when the wearer presses a button. The mechanism often includes different chimes of different pitches to allow the wearer to determine the time within the quarter-hour by hearing alone. Very elaborate versions may even include an automaton like a small singing bird.

• GSTP

General Service Trade Pattern: the pocket watch equivalent of the ATP wristwatch, issued to the British Army during WW2 in very large numbers. Although mostly of Swiss manufacture, an issued model was made in England (albeit in small numbers) by Smiths. Alternative expansions that have been suggested are: General Service Time Piece or General Service Temporary Pattern, but the one above is preferred.

• Guilloche

Also called Engine Turning. This is a very precise engraving technique designed to produce intricate, repetitive designs on a material, including those used on dials.

H

• Hairspring

Also called a balance spring. The very fine metallic spring in a watch movement that allows the balance wheel to recoil. Its length can be adjusted to regulate the watch by making it run faster or slower.

• Hacking seconds

Also called hack seconds or stop seconds. A hacking seconds function allows the wearer to stop the seconds hand, usually by pulling out the crown to a preset stop, while the rest of the mechanism continues to run and keeps the time. It's used to synchronize the watch with someone else's or with a reference time signal without having to adjust the time.

• Hallmark

A form of consumer protection going back 700 years. These are marks stamped on a watch case by a company or some official body that conveys information. They can guarantee the purity of a precious metal used in the case, the quality of the watch as determined by independent inspectors, the country or city of origin, water resistance, serial numbers, manufacturer, year of manufacture, caliber reference, or trademark.

• Hand-wound movement

Also called a manual-wind movement. A mechanical movement that requires periodic winding by hand to function. These often include mechanisms to prevent overwinding.

• Hands

The part of the watch display used to convey information. The hands can come in many forms for aesthetic or practical reasons. In tool watches, it is common for the hands to be shaped as to be difficult to mistake for one another.

• Haute chronologie

Upmarket watchmaking. The term is a play on the fashion industry's haute couture.

• Hard metal

A metal casing made by cementing together titanium and tungsten carbide particles in a high-pressure press. The result is an extremely hard, scratch-resistant material that can be polished with diamond powder. However, it is also very brittle and cannot be repaired if damaged.

• Helium escape valve

Also called a helium release valve. A valve set in a dive watch to prevent it from being damaged by helium. Dive watches with water resistance ratings of over 300 m (1,000 ft) are often worn by mixed-gas divers, who breathe a mixture of helium, hydrogen, and oxygen when working at extreme depths. In between dives and when returning to the surface, the divers sit in a compression chamber filled with the same gas mixture. Because helium molecules are so tiny, they can slip past the watch's gasket and into the case. When the divers leave the chamber and return to sea level pressure, this trapped gas could severely damage the watch, so the one-way helium release valve is used to bleed off the helium when the internal case pressure becomes too great.

• Hertz

Unit for measuring frequencies in cycles per second. It's abbreviated as "Hz."

• Hesalite

A kind of acrylic crystal as used on the Omega Speedmaster Professional and many others.

• Homage watch

A watch that deliberately mimics the design cues of a classic and far more expensive watch without pretending to be that watch, as opposed to a counterfeit or, "replica."

• Horology

The study of time measurement, sundials, clocks, chronometers, watches, and other timepieces, and their construction.

• Hummer

See Tuning Fork

• Hybrid watch

A watch that combines mechanical and smartwatch components.

I-L

• Illumination

A device that lights up the display on the watch on command. This can include simple electric lights; LEDs; photoluminescent materials like Super-LumiNova, LumiNova, or LumiBrite that absorb and re-emit light; phosphorescent materials embedded with radioactive isotopes like radium; and tritium-filled tubes called trasers among others.

• Incabloc

A brand of shock absorber for mechanical watches designed to protect a movement's watch balance staff.

• Index or Indices (plural)

The markings on the dial of a watch used to represent the hours in place of numerals. In higher-end watches, these are usually “applied,” or attached to the dial, rather than printed on. Can be made of steel, gold, onyx etc.

• Integrated bracelet

A watch strap or bracelet that blends into the case and is not detachable. It's most commonly found on fashion watches with a plastic or silicone rubber case and strap.

J

• Jewels

Sapphires or rubies (usually synthetic) that are shaped into gear bearings to reduce friction and increase the life of the movement between servicing.

• Jump hours/minutes

Instead of using analog hands and marks on a dial, jump hours or minutes are a way of producing a digital display using a mechanical movement. In a jumping display, numbered discs rotate behind the watch face and are made visible through an aperture. It's called a jumping display because, where hands move gradually from one mark to the next, the numbers in the aperture remain static until the next minute or hour arrives, then jump ahead.

• Jump seconds

Also called deadbeat seconds, this is a way to make mechanical watch movements shift the seconds hand in discrete one-second intervals in a single tick like a quartz movement. Pendulum clocks have used deadbeat seconds since 1675, but modern mechanical movements usually tick five to ten times per second. Some haute chronologie watches can be made to display deadbeat, though the reason for doing this is simply to show off the complex mechanism needed to achieve it.

K

• Karat

The standard for showing the purity of precious metals indicated by the abbreviation "k." 24k gold is pure, but 18k gold is 75 percent pure. The addition of other metals to gold is required because pure gold is too soft to be practical.

• Keeper

Loops on a watch strap or band designed to keep the surplus end length of the strap in place after its been fastened. ALSO, a watch in a collection that one is reluctant to sell as it is worn regularly and becomes 'a keeper'.

• Kinetic

A technology developed by Seiko for its superquartz movements. Its purpose is to change the wearer's movements into electricity to keep the watch battery fully charged.

L

• Lap timer

A chronograph or stopwatch function that can record the time for individual events in a race, including a runner's laps. When each lap is counted, it returns to zero to time the next lap.

• Lever escapement

Also called an anchor escapement. An escapement that uses an anchor-shaped lever with two pallets to lock and unlock the escapement wheel's teeth.

• Limited edition

A watch that is manufactured and sold in a finite number of units that are often numbered. The purpose is to increase their value.

• Liquid crystal display (LCD)

A flat display on a digital watch that uses liquid crystals sealed in a layer between two transparent plates in a series of sections. When an electric current is applied to a section, its optical properties change and it becomes opaque. This allows it to display digits, letters, and symbols depending on the display's design.

• Lug

Curved protrusions from a wristwatch case for the attachment of a strap or bracelet by means of spring-loaded pins. The inside measurement normally in millimeters will determine the size of the strap or bracelet.

• Luminescence (Lume)

Colloquially referred to as “lume,” luminescence is the glow given off by watch numerals, indices and hands that have been coated with a photoluminescent material (“lumed”). While early watches used radioactive radium to create lume, most modern watches use non-radioactive phosphorescent substances like strontium aluminate.

• Light emitting diode (LED)

A display that uses diodes that emit light when energized. These were used in the first digital watches but were so energy-hungry that they could only be illuminated for a fraction of a second by pressing a button, with the display being invisible the rest of the time.

• Ligne

A historic French unit of measurement used in watchmaking, equivalent to 2.2558291 mm.

M-P

• Main plate

Base plate on which all the other parts of a watch movement are mounted.

• Mainspring

The driving spring of a watch or clock, contained in the barrel.

• Marine chronometer

Highly accurate mechanical or electronic timekeeper enclosed in a box (hence the term box chronometer), used for determining the longitude on board ship. Marine chronometers with mechanical movements are mounted on gimbals so that they remain in the horizontal position is necessary for their precision. Omega Marine Chronometer

• Mark X

Commonly used (erroneously) to describe the IWC WWW (‘Dirty Dozen’) watch. It refers in fact to a watch that was ordered from Smiths late in WW2 and assigned the RAF stores reference of 6B/300. Very few of these watches survive today in any form, but the movement design served as the basis for the very first Smiths wristwatch that appeared on the civilian market after World War 2.

• Measurement conversion

A feature, usually consisting of a graduated scale on the watch's bezel, that lets the wearer translate one type of measurement into another-miles into kilometers, for instance, or pounds into kilograms.

• Mechanical movement

A movement powered by a mainspring, working in conjunction with a balance wheel. Most watches today have electronically controlled quartz movements and are powered by a battery. However, mechanical watches are currently enjoying a resurgence in popularity.

• Movement

The inner mechanism of a watch that keeps time and moves the watch's hands, calendar, etc. Movements are either mechanical or quartz. Otherwise known as a Caliber

• MoD

Ministry of Defence, UK. Used in reference to military watch commissions such as The Dirty Dozen

N

P

• Power Reserve Indicator

A feature that shows when the watch will soon need a new battery or winding. A battery reserve indicator on a quartz watch informs the wearer when the battery is low. Often this is indicated by the seconds hand moving at two or three-second intervals. Seiko's Kinetic watches are quartz watches that do not have a battery (see Kinetic). When a Seiko Kinetic needs to be wound, the seconds hand will also move in two second intervals

• Pusher

A button on a chronograph watch that starts, stops and/or resets the chronograph mechanism. The majority of chronographs have two pushers — one for starting and stopping the mechanism, and another for resetting.

Q-S

• Quartz movement

A movement centred on a quartz crystal cut to a very precise tuning fork design that resonates reliably at 32, 768 Hz when a small voltage is passed through it. Because 32768 is a power of 2 (2^15), cheap logic circuits can be used to produce an output precise to every second that drives a time display (of either analogue or digital form). Quartz movements can be mass produced, making them less expensive than most mechanical movements which require a higher degree of craftsmanship.

R

• Rattrapante Chronograph

Also called a double chronograph or split-seconds chronograph, this adds an additional seconds hand and pusher to the standard chronograph function. The additional seconds hand moves in sync with the standard seconds hand, but stops when the extra pusher is depressed, allowing the user to record two times at once (eg individual versus accumulated lap times). On a second depression of the extra pusher, the additional hand is released and synchronises immediately once again with the continuous seconds hand (hence rattraper, meaning to catch up, recover, recapture).

• Repeater

A high-end complication that chimes to denote the time at the push of a button on the watch case.

• Retrograde

An indicator on a watch dial that forms a segment of a circle, rather than a full circle. When the indicator goes through a full cycle, it resets back to the zero position by moving backward. Often used to indicate hours, minutes or dates.

• Rotating bezel

A bezel (the ring surrounding the watch face) that can be turned. Different types of rotating bezels perform different timekeeping and mathematical functions.

• Rotor

The part of an automatic (or self-winding) mechanical watch that winds the movement's mainspring. It is a flat piece of metal, usually shaped like a semicircle, that swivels on a pivot with the motion of the wearer's arm.

S

• Sapphire crystal

A crystal made of synthetic sapphire, a transparent, shatter-resistant, scratch-resistant substance.

• Screw-lock crown

A crown that can be screwed into the case to make the watch watertight. The time can be set by unscrewing counter-clockwise and pulling out the crown as instruction book indicates; following the adjustment, push and screw clockwise to tighten securely.

• See-through Skeleton back

Case back made of transparent material such as hardened mineral crystal which enables the main parts of the watch to be viewed while working.

• Second

Basic unit of time (abbr. s or sec), corresponding to one 86,000th part of the mean solar day, i.e. the duration of rotation, about its own axis, of an ideal Earth describing a circle round the Sun in one year, at a constant speed and in the plane of the Equator. After the Second World War, atomic clocks became so accurate that they could demonstrate the infinitesimal irregularities (a few hundreths of a second per year) of the Earth's rotation about its own axis. It was then decided to redefine the reference standard; this was done by the 13th General Conference on Weights and Measures in 1967, in the following terms: "The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental state of the atom of caesium 133". Conventionally, the second is subdivised into tenths, hundredths, thousendths (milliseconds), millionths (microseconds), thousand-millionths (nanoseconds) and billionths (picoseconds).

• Second time-zone indicator

An additional dial that can be set to the time in another time zone. It lets the wearer keep track of local time and the time in another country simultaneously.

• Silicon

This metalloid is growing more and more common in watchmaking (mostly in balance springs and escapements), as it does not react to changes in magnetism and temperature like metal does and does not require lubrication. As such, watches with these silicon components are often more accurate, more reliable and more resistant to magnetism than their counterparts using metallic components.

• Shock resistance

As defined by U.S. government regulation, a watch's ability to withstand an impact equal to that of being dropped onto a wood floor from a height of 3 feet.

• Skeleton

Skeleton watch: watch in which the case and various parts of the movement are of transparent material, enabling the main parts of the watch to be seen.

• Skin Diver

Watch for use by skin and scuba divers with water proofing to some degree

• Slide rule

A device, consisting of logarithmic or other scales on the outer edge of the watch face, that can be used to do mathematical calculations. One of the scales is marked on a rotating bezel, which can be slid against the stationary scale to make the calculations. Some watches have slide rules that allow specific calculations, such as for fuel consumption by an airplane or fuel weight.

• Small Seconds

A small sub-dial separate from the main hour and minute function that displays the seconds.

• Smart Watch

Linked to mobile phone or laptop with additional functionality above normal time-keeping

• Solar powered

A watch that uses solar energy (from any light source) to power the quartz movement.

• Stepping motor

The part of a quartz movement that moves the gear train, which in turn moves the watch's hands.

• Stopwatch

A watch with a seconds hand that measures intervals of time. When a stopwatch is incorporated into a standard watch, both the stopwatch function and the timepiece are referred to as a chronograph.

• Sub-dial

A small dial on a watch face used for any of several purposes, such as keeping track of elapsed minutes or hours on a chronograph or indicating the date

T-V

• Tachometer

(aka. "Tachymeter") A feature found on some chronograph watches, measures the speed at which the wearer has traveled over a measured distance. In watchmaking, a timer or chronograph with a graduated dial on which speed can be read off in kilometres per hour or some other unit (see "timer").

• Tank watch

A rectangular watch designed by Louis Cartier. The bars along the sides of the watch were inspired by the tracks of tanks used in World War 1.

• Telemeter

A telemeter determines the distance of an object from the observer by measuring how long it takes sound to travel that distance. Like a tachometer (see "tachometer"), it consists of a stopwatch, or chronograph, and a special scale, usually on the outermost edge of the watch face.

• Timer

Instrument used for registering intervals of time (durations, brief times), without any indication of the time of day.

• Titanium

A metal that is used for some watch cases and bracelets. Titanium is much stronger and lighter than stainless steel. Titanium is also hypo-allergenic.

• Tonneau watch

A watch shaped like a barrel, with two convex sides.

• Tuning Fork Movement

A now-obsolete technology patented by Bulova that centred on a small steel tuning fork with copper wire coils on each tine that could be induced to resonate typically at a reliable 360 Hz when attached to a battery powered transistor. The output from the tuning fork is used to drive an index mechanism that records time. The frequency used is not ultrasonic (compare with quartz movements) so the watches produce a quiet noise that sounds like a hum, hence the colloquial name for them: ‘hummers’.

U

• Unidirectional rotating bezel

An elapsed time rotating bezel (see "elapsed time rotating bezel"), often found on divers' watches, that moves only in a counterclockwise direction. It is designed to prevent a diver who has unwittingly knocked the bezel off its original position from overestimating his remaining air supply. Because the bezel moves in only one direction, the diver can err only on the side of safety when timing his dive. Many divers' watches are ratcheted, so that they lock into place for greater safety.

• UTC

Universal Coordinated Time (UCT), sometimes called Universal Time Coordinated ( UTC ) or Coordinated Universal Time (but abbreviated UTC), and formerly called Greenwich Mean Time.

V

• Vibration

Movement of a pendulum or other oscillating element, limited by two consecutive extreme positions. The balance of a mechanical watch generally makes five or six vibrations per second (i.e. 18,000 or 21,600 per hour), but that of a high-frequency watch may make seven, eight or even ten vibrations per second (i.e. 25,200, 28,800 or 36, 000 per hour).

• Vocab(ulary) Letter

A letter code sometimes seen on military watches either preceding or following an issue number. In some instances (as in the W.W.W.), a letter code has been used to designate a particular manufacturer of the watch.

W-Z

• W.W.W.

Classification of watches issued to the British Army just immediately after WW2. (Expert consensus has it that deliveries were made between May/June 1945 and December 1945). Stands for Wrist Watch Waterproof. The Dirty Dozen were all WWWs. Some waterproof ATP watches were also marked WWW but this may have been in error (although, conversely, examples exist of WWWs having been reclassified as ATPs, possibly due to a loss of waterproofing).

• Water resistant

A watch that is moderately waterproof & will withstand unexpected exposure to water for a very short time (such as splashes). Does not always mean the watch is submersible.

• Winding stem

The button on the right side of the watch case used to wind the mainspring. Also called a "crown."

• World time dial

A dial, usually on the outer edge of the watch face, that tells the time in up to 24 time zones around the world. The time zones are represented by the names of cities printed on the bezel or dial. The wearer reads the hour in a particular time zone by looking at the scale next to the city that the hour hand is pointing to. The minutes are read as normal. Watches with this feature are called "world timers."

X Y Z

• Zulu Time

Zulu (short for "Zulu time") is used in the military and in navigation generally as a term for Universal Coordinated Time (UCT), sometimes called Universal Time Coordinated ( UTC ) or Coordinated Universal Time (but abbreviated UTC), and formerly called Greenwich Mean Time. In military shorthand, the letter Z follows a time expressed in Greenwich Time. Greenwich Time, now called Universal Coordinated Time, is the time at longitude 0 degrees 0 minutes - the prime meridian or longitudinal line that separates East from West in the world geographical coordinate system. This line of longitude is based on the location of the British Naval Observatory in Greenwich, England, near London. "Zulu" is the radio transmission articulation for the letter Z.

Numbers

• 12-hour (24-hour) recorder (or register)

A sub-dial on a chronograph (see chronograph) that can time periods of up to 12 or 24 hours.

• 24-hour movement

A special movement where the hour hand goes around the dial only once a day. This is also called 'Real Military Time'.

• 30-minute recorder (or register)

A sub-dial on a chronograph (see "chronograph") that can time periods of up to 30 minutes