24 hours

A hand, usually on a sub-dial, which completes a rotation in 24 hours and enables the wearer to read the 24-hour time, e.g. 16:00 instead of 4 p.m.

24 hours GMT

A GMT function means that the watch has a second hour hand which completes one full rotation in 24 hours and a 24-hour index on the dial. Particularly useful for wearers who wish to track two time zones.

Alarm

A watch or clock which automatically sounds at a pre-set time.

Annual calendar

Watch which automatically recognizes months with 30 and 31 days and needs to be manually corrected only once a year, on the first of March.

Apnea timing

A watch function which allows free divers to read instantly how much time has passed. A system of seven circles changes colours completely each minute.

Bi-directional rotating bezel

A bezel which can be turned clockwise or anti-clockwise. Rotating bezels can be used to time events, measure elapsed times, or read second or third time zones.

CALENDAR

Date indicator (without synchronisation).

CALIBRE

Technical term denoting the type and dimensions of a movement. It is expressed as a number or in millimetres or lines, the old unit of measurement equivalent to 2.256 mm.

CHRONOGRAPH

Complicated watch with a function for measuring short time periods in addition to its function for permanently displaying hours, minutes and seconds.

CHRONOMETER

Label given to a watch which has undergone precision tests and received a certificate from an official body (COSC).

CO-AXIAL

Escapement invented by Dr G. Daniels and commercialised by Omega for high-precision watches. The escapement uses a low-friction energy transmission system.

CORRECTOR

Manual correction function for a specific element.

COSC

Controle Officiel Suisse des Chronometres [Official Swiss Chronometer Testing Institute].

CROWN

Part used for manual control of the watch (winding, setting the time or date).

Date

The day of the month, displayed in a window on a watch dial typically at the 3 o’clock or 6 o’clock position.

DATE JUMPER

Systems and concepts with an effect on the date change at midnight: Instantaneous: the date changes with a synchronised jump at midnight within a tolerance of 1/- 10 minutes. (Calibre. 1120) Semi-instantaneous: the date moves slowly from around 11.30pm, at midnight the date is still visible. The function concludes with a jump of the disc. Total duration is 45 to 60 min. (Calibres 1128, 1152, 1164) Dragging: slow function, the date changes slowly, starting at around 11pm, at midnight the date is still visible and the function continues without the disc jumping. Total duration is 120 min. (Calibres 1424, 1438, 1538 and quartz calibres with time zones).

Day-Date

Apertures which display both the day of the month (numerical) and the day of the week.

Diamonds

The hardest, most luminous precious stone whose value is calculated according to the 4C criteria: Cut – Carat – Clarity – Color.

DISPLAY

Time indicator Analogue: by comparing the movement of hands in relation to a fixed scale (on the dial) Digital: by reading the digital information directly (LCD-Liquid Crystal Display, LED-Light Emitting Diodes)

E.O.L. (End of Life)

Visual indicator of the end of life of a battery by successive jumps of the seconds hand of 4 seconds, or a flashing digital display.

FLY-BACK HAND

A fly-back chronograph differs little from a normal chronograph. The main difference is an additional mechanism for driving a second hand called the fly-back hand.

FREQUENCY

Number of oscillations per second, the unit being Hertz (Hz) 1 Hz = 1 oscillation per second – 32780 Hz = 32780 oscillations per second (a common frequency in quartz watches)Oscillator: watch (or clock) part which makes a regular movement ensuring the division and measurement of time (pendulum, balance spring or quartz). Oscillation: the complete swing of an oscillator (the sum of 2 vibrations)Vibration: a swing between 2 extreme positions of the oscillator.

GMT

Greenwich Mean Time. Average time from zone 0, introduced on 01.11.1884, known scientifically as UTC (Universal Time Co-ordinated) and used as universal standard time.

HELIUM ESCAPE VALVE

Decompression system which allows helium to escape from inside the watch when the watch is worn for professional use in pressure chambers. (Long-term underwater work, crude oil exploration, etc.)

JUMPING HOURS

Function allowing digital display of the hours in a window at 12 o’clock and the display of minutes by a hand. Each revolution of the minutes hand triggers an immediate jump in the hour window.

Limited edition

A series of watches whose production run is determined in advance of production and limited to a set number.

Moonphase

A function which tracks the phases of the moon. One lunar cycle is 29 days, 12 hours, 44 minutes and 2.8 seconds and includes four phases: new moon, first quarter, full moon, last quarter.

MULTIFUNCTION

Watch with digital (or combined analogue/digital) display with several functions, such as alarm, chronograph, countdown timer etc. with optional display of the required mode.

Numbered edition

A watch series each of whose pieces is numbered sequentially in the order it is produced.

PERPETUAL CALENDAR

Memory function that respects all date changes, including 29th February in leap years (until 28/02/2100).

POWER RESERVE

Period during which a watch or instrument can function on its own energy reserves (spring, battery etc.)

Power reserve indicator

A dial or gauge on the watch which indicates how much time remains before the mainspring must be wound or the battery changed.

Pulsometer

A watch with a scale to measure the number of heartbeats per minute.

RAPID DATE CORRECTOR

Function for correcting the date (months of 28, 29 or 30 days) without altering or stopping the watch’s operation.

Regatta timing

A watch function which allows the easy reading of the critical regatta time intervals at glance.

SCREW-IN CROWN

Self-locking crown, screwed into the tube of the case, used for highly water resistant divers’ watches.

SEMI-PERPETUAL CALENDAR

Memory function that respects all date changes except for 29th February in leap years (manual correction).

Small seconds

A hand on a sub-dial which tracks seconds; typically it completes a full rotation in one minute.

SPLIT-SECONDS (RATTRAPANTE)

In appearance a split-seconds chronograph is only slightly different from a standard chronograph. Its special feature is the additional device which drives a second center-sweep seconds hand called the split-seconds hand. When the chronograph mechanism is activated the two superimposed hands rotate around the dial. If the split-seconds pusher is pressed the split-seconds hand will stop, allowing the user to read the intermediate time. Meanwhile the chronograph seconds hand continues to turn. If the same pusher is pressed again, the split-seconds hand will catch up with the chronograph hand and continue to rotate in perfect unison with it. This operation can be re-peated as many times as desired.

STOP SECONDS (function)

A braking system on the balance-spring which stops the seconds hand when the crown is pulled. This allows the seconds hand to be synchronised with an external time signal when setting the time (crown in position 3).

Tachymeter

A tachymeter is an instrument for measuring speed. It is a chronograph with a graduated dial on which speed can be read off in kilometres per hour based on 1000 metres distance.

THERMO-COMPENSATION

Electronic procedure for fine adjusting a quartz watch (comparable to inhibition), using a thermal sensor which allows better compensation of differences in rate resulting from temperature fluctuations experienced by the watch in use. Degree of precision: better than 1 second per month.

TIME ZONE (Function)

Display of regional time (or universal time) by an independent movement of the hour hand, without affecting the watch’s precision (minutes and seconds).

TIME ZONES

Division of the Earth’s surface into 24 equal zones which determine the corresponding regional time. Zone “0″ coincides with the Greenwich Meridian and corresponds with universal time (or GMT : Greenwich Mean Time).

TOURBILLON

An ingenious principle which solves the tricky problem of adjusting mechanical watches. All the components of the escapement (escape wheel, pallets wheel, pallets fork and balance spring) are grouped together in a small mobile cage which makes one rotation per minute. This eradicates any differences in rate due to different positions of the watch and thus guarantees optimal precision for a mechanical watch.

Transparent case back

A transparent case back – frequently made of sapphire – makes it possible to see the movement inside the watch.

Unidirectional rotating bezel

A bezel which turns in only one direction. Particularly useful for divers who cannot accidentally rotate it in the wrong direction, which could cause them to miscalculate their dive times. A rotating bezel makes it easy to measure elapsed time.

WATER RESISTANCE

A measure of the watch’s capacity to resist penetration by water or condensation.