A collection of terms related to horology.


Accuracy Watches and clocks are among the oldest and most precise mechanical devices. A movement that shows a 30-second departure from true time in a 86,400 second day has a mathematical error of 0.035%. In other terms, it is 99.965% accurate. Officially certified chronometers can achieve an error of less than 0.005%.

Adjustment  The fine-tuning of a movement in various positions and temperatures. There are various degrees of adjustment; an ordinary watch is timed crown up and dial up, and adjusted for a maximum rate variation of 30 seconds a day between the two positions. For precision adjustment the movement is timed in five positions and at three different temperatures, usually 4°, 20°, and 38°C.

Alarm  One of the earliest complications of mechanical timepieces, known since the 16th century. Today's wrist alarms can be automatic, manually wound or quartz regulated.

Amplitude  The maximum angle from dead centre described by a balance-wheel or pendulum.

Analogue indications The conventional way of showing the time with hands moving around the dial.

Annual calendar A watch showing the calendar without correction for a year, from 1 March to 28 February.

Anti-magnetic A watch is considered anti-magnetic if it continues to function in a magnetic field of 4800 A/m (Amperes per meter) with a maximum rate variation of 30 seconds a day. To this end, most of the essential components of the regulating organ (the pallet-lever, escape wheel, roller, balance-staff, wheel and spring) are made of metals that cannot be magnetized or only with difficulty. To increase resistance to magnetic fields, some manufacturers surround the movement with a shell of conductive metal, such as soft iron, which prevents magnetic fields building up inside it. Magnetized movements can be neutralized with the help of a degaussing coil.


Aperture Where the date is displayed within the dial of the watch

Assay The quantitative analysis to determine the proportion of precious metals in an alloy.

Assembly  Putting together components into a functioning watch.

Atmosphere (Atm) A measure of the water resistance of a watch.

Automatic lathe A term describing a wide range of automatic machine-tools for milling preliminary watch parts.

Automatic winding  A device which uses the motions of the wrist to wind the mainspring of a mechanical watch. Abraham-Louis Perrelet is credited as the inventor of the portable self-winding timepiece in the 18th century. The first series-produced automatic wristwatches date back to the Englishman, John Harwood, who patented a watch wound by a swinging weight in1923. In 1930 Rolex introduced a rotor-based winding system, in which the mainspring was wound in one direction of rotation. At the same time a range of other "jiggle-action" devices appeared, in which the movement itself acted as the oscillating weight, moving back and forth inside or with the case to wind the spring in both directions of motion. With the invention of the rotor mounted on ball-bearings by Eterna in 1948, all conditions were met for the future of self-winding systems. Automatic movements reached their production peak in the mid-seventies.