A calendar complication that automatically accounts for months with 30 or 31 days. It normally needs correcting only once a year, at the end of February.

A watch or movement designed to resist the effects of magnetic fields, which can otherwise affect the accuracy of a mechanical watch.

A mechanical movement that winds its mainspring using the motion of the wearer's wrist, usually through a rotating weight known as a rotor.

A unit used to describe water resistance. As a guide, 10 ATM is approximately equivalent to 10 bar or 100 metres of static pressure.

The oscillating component that works with the balance spring to regulate the rate of a mechanical watch.

A very fine spring attached to the balance wheel. Its controlled expansion and contraction helps regulate timekeeping.

The ring surrounding the watch crystal. It may be decorative or functional, for example on dive, GMT or chronograph watches.

Steel that has been heat-treated until a blue oxide layer forms on its surface, commonly used for hands and screws.

A plate secured to the movement that supports wheels, pinions and other components.

A polished gemstone with a smooth, rounded surface rather than facets, sometimes used to decorate a watch crown.

The outer body of the watch that houses and protects the movement.

The rear of the watch case. It may be solid or fitted with a transparent crystal to reveal the movement.

A hard, lightweight and highly scratch-resistant material used for cases, bezels and bracelet components.

A watch with a stopwatch function that can start, stop and reset elapsed time independently of the normal time display.

A precision timepiece or movement that has met defined accuracy testing standards from an authorised testing body.

A decorative guilloché pattern made up of small intersecting pyramidal shapes, often described as a hobnail pattern.

An escapement developed to reduce sliding friction within a mechanical movement. It is closely associated with OMEGA mechanical watches.

A finely engineered component used in some chronograph movements to coordinate the start, stop and reset functions.

Any watch function beyond the basic display of hours, minutes and seconds, such as a chronograph, calendar, moonphase or repeater.

The Contrôle Officiel Suisse des Chronomètres, the Swiss body that tests movements against recognised chronometer performance standards.

A traditional decorative finish made up of parallel wave-like stripes, frequently applied to mechanical watch movements.

A function that counts down towards a predefined point in time, often associated with regatta or sailing watches.

The external control normally used to wind a watch and set the time, date or other functions.

A folding clasp that opens on hinged sections, allowing a strap or bracelet to remain looped around the wrist when opened.

A display that shows the time in two different time zones simultaneously.

The mechanism that transfers energy from the gear train to the regulating organ of a mechanical watch in controlled increments.

A transparent caseback, usually made from sapphire crystal, allowing the wearer to view the movement.

A chronograph that can be reset to zero and restarted immediately with a single press while it is running.

The rate at which a movement's regulating organ oscillates, commonly expressed in hertz or vibrations per hour.

A dial with a graduated colour effect, usually darker around the edges and lighter towards the centre.

A watch capable of displaying a second time zone, typically using an additional 24-hour hand and scale.

A highly complex striking complication that automatically sounds the hours and quarters as they pass.

A decorative engraving technique using repeated geometric lines or patterns on a dial, case or movement component.

A unit of frequency. One hertz is one complete cycle per second.

A term commonly used for mechanical movements operating at a relatively high frequency, often 36,000 vibrations per hour or 5 Hz.

A shock-protection system designed to protect the delicate balance staff pivots in a mechanical movement.

A bracelet designed as a visual and structural continuation of the watch case rather than appearing as a separate attachment.

Synthetic rubies or similar hard materials used as low-friction bearings at key pivot points within a mechanical movement.

A complication that displays the hour through an aperture and changes instantly from one hour to the next.

A date display using two separate numeral discs or windows to create a larger, easier-to-read date indication.

The projections on a watch case that connect the case to a strap or bracelet.

The distance between the lugs, normally measured in millimetres, which determines the correct strap or bracelet width.

Luminous material applied to hands, hour markers or bezels to improve legibility in low light.

A mechanical movement that requires the wearer to wind the mainspring manually using the crown.

A movement developed and produced substantially by the watchmaker or group responsible for the finished watch.

A certification associated with stringent performance, precision, water resistance and magnetic-resistance testing carried out under METAS-approved procedures.

The Swiss Federal Institute of Metrology. In watchmaking it is associated with Master Chronometer testing and certification.

A clasp system that allows small changes to bracelet or strap length without removing or adding links.

A striking complication that sounds the hours, quarter-hours and minutes on demand.

A complication displaying the changing phases of the moon, typically based on the approximately 29.5-day lunar cycle.

A dial opening that reveals part of the movement, most commonly the oscillating balance assembly.

Also called a rotor. A weighted component in an automatic movement that rotates with wrist movement and winds the mainspring.

A highly sophisticated calendar complication designed to account automatically for varying month lengths and leap years.

The length of time a mechanical watch can continue running from a fully wound mainspring.

A display showing approximately how much running time remains before a mechanical watch requires winding.

A battery-powered movement regulated by the vibration of a quartz crystal, known for high accuracy and low maintenance.

A mechanical complication that strikes the time acoustically on demand.

A display in which a hand travels along an arc or scale and then jumps back to its starting position at the end of its cycle.

The oscillating weight inside an automatic movement that helps wind the mainspring.

A layered dial construction in which the upper dial has cut-outs that reveal luminous or contrasting material beneath.

A highly scratch-resistant synthetic crystal commonly used to protect the dial and, on exhibition casebacks, the movement.

A crown that screws securely into the case to help improve resistance to water and dust.

Chronograph pushers that can be locked with threaded collars to help improve water resistance and prevent accidental operation.

A balance spring manufactured from silicon, valued for its resistance to magnetism, corrosion and wear.

A dial or watch construction that exposes substantial parts of the movement so its mechanics can be viewed from the front.

A movement whose plates and bridges are cut away to reveal as much of the mechanism as possible while retaining structural integrity.

Also known as a rattrapante. A chronograph with two superimposed timing hands that can measure intermediate times or two events that start together.

Grand Seiko's proprietary movement technology combining a mechanical mainspring and gear train with an electronic regulator, producing a smooth gliding seconds hand.

A dial finish with fine radial brushing that reflects light outwards from the centre.

A scale, commonly paired with a chronograph, used to calculate average speed over a known distance.

TUDOR's rapid-adjustment clasp system, allowing the wearer to make small bracelet length adjustments without tools.

A rotating cage containing the escapement and regulating organ, originally developed to reduce the effect of gravity on positional timekeeping errors.

A bezel designed to rotate in only one direction, widely used on dive watches so accidental movement can only indicate less remaining dive time.

A chronograph coupling system that engages the chronograph mechanism vertically, helping provide a smooth start to the chronograph seconds hand.

A rating indicating the level of pressure a watch case is designed to withstand under specified test conditions. It should not be interpreted as a guarantee for all real-world activities.

A specialist distortion-free polishing technique closely associated with Grand Seiko, used to create exceptionally flat, mirror-finished surfaces.