Clock Glossary: Clockmaking Terms from A to Z
Clockmakers and collectors use a vocabulary all their own, from arbors and pallets to fusees and spandrels. This glossary explains the most common terms for clock parts, movements, escapements, striking and chiming, cases and dials, and modern electric, quartz and atomic timekeeping in plain English. Where a term has its own page on clocks.fyi, follow the link to read more.
3
30-hour clock: A clock that needs winding every day. It runs a little longer than a day so a late winding does not stop it. More →
4
400-day clock: Another name for an anniversary clock, which can run for about a year on one winding thanks to its slow torsion pendulum. More →
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8-day clock: A clock that runs for about a week on one winding. The extra day gives the owner a margin, so the clock can be wound on the same day each week without running down. More →
A
Alarm: A mechanism that sounds a bell, buzzer or tone at a time the owner sets. Mechanical alarms have their own spring or weight, released when the alarm setting lines up with the hour wheel.
Anchor escapement: An escapement for pendulum clocks whose pallets look like an upside-down ship's anchor. Developed in England in the 17th century, it lets the pendulum swing in a small arc and became standard in longcase and tower clocks. It is also called the recoil escapement. More →
Anniversary clock: A clock, usually under a glass dome, regulated by a torsion pendulum that slowly twists back and forth. Its slow beat lets it run for about a year on one winding. More →
Arbor: The axle or shaft in a clock movement that carries a wheel, a pinion or both. Its thin ends, the pivots, turn in holes in the plates.
Arch dial: A dial with a half-round section added above the square part. The arch often holds a moon-phase display, a strike/silent dial or the maker's name.
Atmos clock: A clock made by Jaeger-LeCoultre that winds itself from small changes in air temperature and pressure, using a sealed capsule that expands and contracts. More →
Atomic clock: A clock that keeps time by measuring the steady natural frequency of atoms, such as cesium. Atomic clocks are the most accurate timekeepers ever made, and the official length of the second is based on one.
Automaton: A moving figure or scene worked by the clock mechanism, such as a bird that sings, a figure that strikes a bell or a parade that appears on the hour. More →
B
Balance spring: A fine coiled spring attached to a balance wheel. It makes the balance swing back and forth at a steady rate and is often called the hairspring.
Balance wheel: A weighted wheel that swings back and forth to regulate a clock or watch that has no pendulum. Paired with a balance spring, it works in any position, which makes it suitable for portable timepieces.
Banjo clock: An American wall clock shaped roughly like a banjo, with a round dial, a narrow waist and a box below. It grew out of Simon Willard's patent timepiece. More →
Barrel: A drum inside a clock. In a weight-driven clock the cord or cable winds around it; in a spring-driven clock it is a sealed drum that holds the mainspring.
Beat: The rhythm of the tick and the tock. A clock is in beat when the two sounds are evenly spaced; an out-of-beat clock sounds lopsided and may stop.
Bezel: The ring, usually brass or wood, that holds the glass over the dial. On many clocks it is hinged so it can be opened for winding and setting the hands.
Bob: The weight at the bottom of a pendulum. Raising or lowering it changes the pendulum's effective length, and so the clock's rate. More →
Bolt and shutter: A type of maintaining power on early clocks. Before winding, the owner pulls a cord or lever that opens shutters over the winding holes and, at the same time, pushes on the train so the clock keeps going while being wound.
Bracket clock: A spring-driven table or shelf clock, usually with a carrying handle on top. The name comes from the wall brackets some of them stood on. More →
Brocot escapement: A version of the deadbeat escapement often fitted to French clocks, with half-round pallets of steel or jewel. It is frequently mounted on the front of the dial where it can be seen working. More →
Brocot suspension: An adjustable pendulum suspension found on many French clocks. A small square above the XII lets the owner raise or lower the pendulum with a key, adjusting the rate without touching the bob.
Bushing: A small brass sleeve fitted into a plate to replace a pivot hole that has worn oval. Rebushing worn holes is one of the most common clock repairs.
C
Calendar aperture: A small window in a dial that shows the day of the month on a turning disc or ring behind it.
Cannon pinion: A short tube that fits on the center arbor and carries the minute hand. It is held by friction, so the hands can be turned to set the time without damaging the movement, and it drives the motion work.
Carriage clock: A small, portable spring-driven clock, usually in a brass case with glass panels and a handle on top. They were made in large numbers in France in the 19th century. More →
Case: The housing that protects a clock's movement and pendulum and gives the clock its style. Cases range from simple wooden boxes to carved, inlaid or gilded showpieces.
Center wheel: In many clocks, the wheel at the middle of the movement that turns once an hour. Its arbor passes through the dial and carries the minute hand by way of the cannon pinion.
Chapter ring: The ring on a dial marked with the hour numerals and minute divisions. On early brass dials it is a separate silvered ring fixed to the dial plate.
Chime: A tune played on several bells, rods or gongs, usually at each quarter hour. A chiming clock plays a melody; a striking clock simply counts the hours. More →
Chime train: In a three-train clock, the separate set of wheels and its own spring or weight that drives the chime. The other two trains handle timekeeping and hour striking.
Chronometer: A highly accurate timekeeper, especially a marine chronometer used at sea to find longitude. Most marine chronometers use a detent escapement and sit in gimbals inside a wooden box. More →
Click: A small pawl that drops into the teeth of a ratchet wheel so a spring or weight can be wound one way but cannot unwind backward. The clicking sound when winding comes from it.
Click spring: The small spring that keeps the click pressed against the ratchet wheel. A broken click spring can let a mainspring unwind suddenly.
Compensation pendulum: A pendulum built so that heat and cold do not change its effective length. Gridiron and mercury pendulums and rods of Invar are common solutions. More →
Comtoise clock: A weight-driven clock from the Franche-Comté region of France, often with a long, decorated pendulum and a case or wall mount. More →
Conical pendulum: A pendulum whose bob travels in a circle instead of swinging back and forth, so it turns steadily rather than ticking. More →
Count wheel: A wheel with notches at uneven spacings that controls how many times a clock strikes. Because it simply moves on to the next hour, it can fall out of step with the hands and cannot repeat a strike. Also called a locking plate.
Crown wheel: The escape wheel of a verge escapement, with saw-like teeth standing up around its rim like the points of a crown. More →
Crutch: The arm that hangs down from the pallet arbor and engages the pendulum rod. It passes each push from the escapement to the pendulum and can be bent or adjusted to put a clock in beat.
Crystal oscillator: The heart of a quartz clock: a small quartz crystal that vibrates at a very steady rate when an electric current is applied to it. A circuit counts the vibrations to measure time. More →
Cuckoo clock: A wall clock, traditionally from Germany's Black Forest, in which a small bird pops out and calls the hours. The call comes from two small pipes, each blown by its own bellows.
Cycloidal cheeks: Curved metal guides placed on either side of a pendulum's suspension in Christiaan Huygens's early clocks. They bent the pendulum's path so that wide and narrow swings took the same time. More →
D
Deadbeat escapement: A refined form of the anchor escapement associated with George Graham. Its pallets are shaped so the escape wheel does not push backward, so the seconds hand stops dead instead of recoiling. It became the choice for precision pendulum clocks. More →
Detent escapement: The escapement used in most marine chronometers. A light catch, the detent, holds the escape wheel and releases it once per swing of the balance, leaving the balance free for most of its motion. More →
Dial: The face of a clock, showing the hours and minutes. Dials may be engraved brass, painted iron or wood, enamel, paper or printed metal.
Drop: The short free movement of the escape wheel after a tooth leaves one pallet and before it lands on the other. Even drop on both sides is part of putting an escapement in good order.
Duration: How long a clock runs on one winding, such as 30 hours, 8 days or a month. More →
E
Electric clock: Any clock that uses electricity, either to drive the hands directly, to keep a spring or pendulum going, or to send time signals from a master clock to other dials.
Endless rope: An arrangement credited to Christiaan Huygens in which a single loop of rope or chain carries one weight that drives the clock, often both its trains. It keeps the clock going while it is wound, and it is common in 30-hour longcase clocks.
Equation clock: A clock that shows the equation of time, either on an extra dial or with a second minute hand, so the owner can compare clock time with sundial time. More →
Equation of time: The difference between apparent solar time, as shown by a sundial, and the even mean time a clock keeps. It changes through the year, and on some dates the two can be apart by about a quarter of an hour. More →
Escape wheel: The toothed wheel at the end of the going train that the pallets catch and release in turn. Each release lets the clock advance by a tiny, regular step. More →
Escapement: The mechanism that lets a clock's power out in small, regular steps. It locks the train, releases it one tooth at a time in step with the pendulum or balance, and gives the oscillator a small push to keep it going. More →
F
Finial: A decorative knob, ball or spire on top of a clock case, usually of brass or wood. Longcase clocks often have several on the hood.
Fly: A small fan on the striking train that spins as the clock strikes. Air resistance on its vanes keeps the strike at an even, unhurried pace.
Foliot: A horizontal bar with adjustable weights at each end, mounted on the verge of the earliest mechanical clocks. Moving the weights outward slows the clock; moving them in speeds it up. More →
Free pendulum: A pendulum kept as free as possible from driving the clock, as in the Shortt clock, where a second, slave clock does the counting and gives the master pendulum only a gentle push at intervals. More →
Fretwork: Pierced decorative woodwork or metalwork, often on top of a longcase hood or on the sides of a bracket clock, where it can also let the sound of the bell out.
Fusee: A cone-shaped pulley linked to the mainspring barrel by a chain or gut line. When the spring is fully wound it pulls on the narrow end; as it runs down it pulls on the wider end, which evens out the force reaching the clock.
G
Gathering pallet: In rack striking, a small arm on the strike train that gathers up the rack one tooth for each blow of the hammer, then locks the train when the rack is fully gathered.
Gimbals: A pair of pivoted rings that let a marine chronometer stay level as a ship rolls. More →
Going barrel: A mainspring barrel with teeth around its rim that drives the clock directly, without a fusee.
Going train: The set of wheels and pinions that does the timekeeping, running from the power source to the escapement. It is also called the time train.
Gong: A coiled or straight steel wire or rod that gives a ringing tone when struck by a hammer. Many later clocks use gongs instead of bells.
Grande sonnerie: A striking system that sounds the hour and then the quarters at every quarter hour, so the full time is announced four times an hour.
Grandfather clock: The popular name for a longcase clock, a tall floor-standing clock with a long pendulum inside its case.
Grasshopper escapement: An unusual escapement devised by John Harrison, with hinged pallets that kick in and out with very little sliding friction, so it needs almost no oil. More →
Gravity escapement: An escapement in which the pendulum is pushed by small arms lifted a fixed amount by the clock and then allowed to fall, so the push stays the same whatever happens in the gears. Edmund Denison's double three-legged gravity escapement drives the Great Clock at Westminster. More →
Gridiron pendulum: A compensation pendulum devised by John Harrison, made of parallel rods of two metals, such as steel and brass, arranged so their expansion in heat cancels out. More →
H
Hairspring: A common name for the balance spring, the fine coiled spring that controls the swing of a balance wheel.
Hammer: The arm that strikes a bell, gong or rod. It is lifted by pins on a wheel in the striking train and then drops or springs back to sound the blow.
Hood: The top section of a longcase clock case that encloses the dial and movement. It usually slides forward or lifts off so the movement can be reached.
Horology: The science and craft of measuring time and of making and repairing clocks and watches.
Hour strike: Striking that counts out the hour, one blow at one o'clock up to twelve at twelve. Many clocks also give a single blow at the half hour.
Hour wheel: The wheel in the motion work that carries the hour hand and turns once every 12 hours.
I
IIII: The form of the number four used on many clock dials in place of IV. Several explanations are offered, from visual balance to tradition, and no single reason is agreed.
Impulse: The small push the escapement gives the pendulum or balance on each swing to replace the energy it loses to friction and air. More →
In beat: Said of a clock whose tick and tock are evenly spaced, meaning the escapement releases at the same point on either side of the pendulum's swing.
Invar: A nickel-steel alloy that barely changes length with temperature, developed by the Swiss-born physicist Charles-Edouard Guillaume. It is used for precision pendulum rods.
Isochronism: The property of an oscillator, such as a pendulum or balance, of taking the same time for each swing whether the swing is wide or narrow. More →
J
Jewel: A bearing made of a hard stone, today usually synthetic ruby, used to reduce friction and wear at pivots and pallets.
L
Lantern clock: An early type of weight-driven domestic clock with a brass case, a bell on top and a single hour hand, made in England from the 17th century. More →
Lantern pinion: A pinion made of short steel wires, called trundles, set between two end discs instead of cut teeth. Lantern pinions are common in mass-produced American clocks.
Lenticle: A small glass window in the trunk door of a longcase clock through which the pendulum bob can be seen swinging.
Lever escapement: The escapement used in most mechanical watches and many alarm and carriage clocks. A pivoted lever with two pallets passes impulses from the escape wheel to the balance, which swings freely most of the time. More →
Locking plate: Another name for a count wheel, the notched wheel that controls the number of blows in older striking clocks.
Longcase clock: A tall, floor-standing clock with the weights and long pendulum inside a wooden case, known in America as a tall case clock and popularly as a grandfather clock.
Lunette: The arched upper part of a dial, often painted and often holding a moon-phase display.
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Mainspring: A long coiled strip of spring steel that powers a spring-driven clock or watch as it slowly unwinds.
Maintaining power: A device that keeps the clock going while it is being wound, since winding would otherwise take power off the train. Bolt-and-shutter work, endless ropes and Harrison's spring-based maintaining power are examples.
Mantel clock: A clock made to stand on a shelf or fireplace mantel. Most are spring-driven, and many strike or chime.
Marriage: A collector's term for a clock whose case, movement or dial did not start life together but have been combined later.
Master clock: A precision clock that controls a network of other clocks, called slave clocks or secondary clocks, by sending them regular electrical signals. More →
Mean time: Time measured by an imaginary sun moving at an even pace through the year, so every day is exactly the same length. Clocks keep mean time; sundials show apparent solar time.
Mercury pendulum: A compensation pendulum associated with George Graham in which the bob is a jar of mercury. As heat lengthens the rod, the mercury expands upward, keeping the effective length the same. More →
Minute wheel: The wheel in the motion work that links the cannon pinion to the hour wheel, helping to reduce the minute hand's motion twelve times for the hour hand.
Month clock: A clock that runs about a month on one winding, usually with a heavier weight or stronger spring and extra wheels in the train.
Moon-phase dial: A display, often in the arch of a dial, showing the moon's changing phase. A common design uses a disc painted with two moons that turns once every two lunar months. More →
Motion work: The small set of wheels behind the dial that makes the hour hand turn once for every twelve turns of the minute hand.
Movement: The working mechanism of a clock, apart from the case and dial.
Musical clock: A clock that plays tunes, usually on bells or a pinned cylinder, at set times or on demand. More →
Mystery clock: A clock whose hands seem to float with no visible connection to any mechanism, often worked by hidden glass discs or a concealed pendulum. More →
O
Ogee clock: A 19th-century American shelf clock in a rectangular case framed by an S-curved molding, known as an ogee. Ogee clocks were made in large numbers in Connecticut.
Oil sink: A small cup cut around a pivot hole on the outside of a plate. It holds a drop of oil against the pivot by capillary action.
P
Pallets: The two surfaces on the escapement that catch and release the teeth of the escape wheel in turn and pass on the impulse to the pendulum or balance. More →
Passing strike: The simplest form of striking: a single blow on a bell or gong as each hour passes, tripped by the going train rather than a separate striking train.
Pendulum: A weight on a rod or cord that swings back and forth at a steady rate set mostly by its length. Since the 1650s it has been the main timekeeping element in most mechanical clocks. More →
Petite sonnerie: A striking system that sounds the quarters at each quarter hour but strikes the hour only once, at the hour itself.
Pillar: One of the posts that hold the front and back plates of a movement apart. Shapes of pillars are one of the details used to date early clocks.
Pillar and scroll clock: An early 19th-century American shelf clock with slender pillars and a scrolled top, associated with Eli Terry. More →
Pin wheel: A wheel in the striking train with pins around its rim that lift the hammer, one pin for each blow.
Pin-pallet escapement: An inexpensive form of lever escapement with upright steel pins instead of jeweled pallets, widely used in alarm clocks and low-cost watches.
Pinion: A small gear with few teeth, called leaves, usually cut from steel and driven by a larger wheel.
Pivot: The thin, polished end of an arbor that turns in a hole in the plate.
Plates: The two metal sheets, front and back, that hold the wheels of a clock movement between them.
Plinth: A raised base. On a longcase clock the word is also used for the small blocks on the hood that support the finials.
Pull repeat: A cord or lever that makes a clock strike again on demand, so the owner can hear the time in the dark. It needs rack striking, because a count wheel cannot repeat.
Q
Quarter strike: Striking or chiming at each quarter of the hour, as opposed to striking only the hours.
Quartz clock: A clock regulated by a vibrating quartz crystal. An electronic circuit counts the crystal's vibrations and drives a digital display or a small stepping motor that moves the hands. More →
R
Rack striking: A striking system in which a toothed rack drops onto a stepped cam, the snail, to decide the number of blows. Introduced in England in the 1670s and credited to Edward Barlow, it cannot get out of step with the hands and allows a clock to repeat.
Radio-controlled clock: A clock that sets itself from a broadcast time signal. In the United States, these clocks listen to NIST's station WWVB in Colorado.
Ratchet wheel: A toothed wheel on a winding arbor that works with the click so the clock can be wound but cannot run backward.
Rate: How much a clock gains or loses over a set period, such as seconds per day. Regulating a clock means adjusting its rate.
Rating nut: The threaded nut under a pendulum bob. Screwing it up shortens the pendulum and makes the clock run faster; screwing it down makes it slower. More →
Recoil escapement: Another name for the anchor escapement, so called because the escape wheel is pushed slightly backward at the end of each swing, which can show as a small backward jerk of the seconds hand. More →
Regulator: Two related meanings: a very accurate clock used as a standard for setting others, often with a deadbeat escapement and a compensated pendulum; or the part of any clock that is adjusted to make it run faster or slower. More →
Remontoire: A device that rewinds a small spring or weight at short intervals so the escapement always receives the same force, no matter how the main power source varies.
Repeater: A clock or watch that strikes the last hour, and sometimes the quarters or minutes, when a cord, button or lever is pulled.
S
Seconds bit: A small separate dial, often just above the center of a longcase dial, that shows the seconds.
Seconds pendulum: A pendulum just under a meter (about 39 inches) long that takes one second for each swing. It is standard in longcase clocks and many regulators. More →
Ship's bell clock: A clock that strikes the time in the bells used at sea, from one to eight bells, following the half hours of a ship's four-hour watches. More →
Sidereal time: Time measured by Earth's rotation relative to the stars rather than the sun. A sidereal day is about four minutes shorter than a solar day, and observatories kept special clocks running on it.
Skeleton clock: A clock with its movement on show, the plates cut away to a lacy frame, usually displayed under a glass dome. More →
Slave clock: A clock that does not keep time itself but is advanced by signals from a master clock. More →
Snail: A stepped, snail-shaped cam that turns with the hour wheel in rack striking. The depth of each step sets how far the rack falls, and so how many times the clock strikes.
Spandrel: A decorative corner ornament on a clock dial, cast in brass on early brass dials and painted on later painted dials.
Stepping motor: The tiny electric motor in an analog quartz clock that moves the hands forward one step for each pulse from the circuit, which is why many quartz second hands tick once a second. More →
Stopwork: A device on a spring-driven clock that limits how far the mainspring can be wound and let down, so only its most even turns are used.
Strike/silent: A lever or small dial that lets the owner turn a clock's striking or chiming off and on.
Striking train: The separate set of wheels, with its own spring or weight, that drives the hammer to strike the hours.
Sundial: An instrument that tells apparent solar time from a shadow cast by a pointer, the gnomon, onto a marked surface. More →
Suspension spring: The short, thin strip of spring steel from which a pendulum hangs. It flexes with each swing and is a common part to replace when a clock will not run.
Synchronous clock: An electric clock driven by a motor that runs in step with the alternating current from the power grid, 60 cycles a second in North America. It is only as accurate as the grid's frequency. More →
T
Tall case clock: The usual American term for a longcase or grandfather clock.
Time ball: A large ball dropped from a mast at an exact time each day so that ships in port could check their chronometers. More →
Ting-tang: A simple quarter strike on two bells or gongs: one ting-tang pair at the first quarter, two at the half and three at the three-quarters.
Torsion pendulum: A pendulum that hangs from a thin wire or strip and rotates slowly one way and then the other instead of swinging. It is used in anniversary clocks. More →
Tourbillon: A rotating cage that carries the balance and escapement, turning continually so that errors caused by the watch's position average out. It was invented by Abraham-Louis Breguet and is mainly found in high-end watches.
Tower clock: A large clock in a church, town hall or other public tower, often called a turret clock, with dials outside and a bell to strike the hours. More →
Train: A series of wheels and pinions that pass power through a clock. Most clocks have a going train for timekeeping, and many also have striking and chiming trains.
Trunk: The long middle section of a longcase clock case between the hood and the base, containing the weights and pendulum. It is also called the waist.
V
Verge escapement: The earliest mechanical escapement, probably invented in 13th-century Europe. Two pallets on an upright rod, the verge, catch the teeth of a crown wheel in turn, with the swing controlled first by a foliot or balance and later by a short pendulum. More →
Vienna regulator: A weight- or spring-driven wall clock in a glazed case, first made in Vienna, known for its long pendulum and fine timekeeping. More →
W
Warning: A moment a few minutes before a clock strikes when the striking train is partly released and then caught again, ready to strike exactly on the hour. It often makes an audible click.
Weight-driven: Powered by a falling weight on a cord, cable or chain rather than by a spring. Winding raises the weight again.
Westminster quarters: The four-bell quarter chime heard from Big Ben, which originated at Great St Mary's Church in Cambridge and is also called the Cambridge Quarters. It builds through the hour, from four notes at the first quarter to sixteen before the hour strikes. More →
Wheel: Any of the larger toothed gears in a clock train. A wheel usually drives a pinion on the next arbor.
Winding arbor: The squared end of an arbor that a key or crank fits onto to wind a spring or raise a weight. Each train usually has its own. More →
Winding hole: A hole in the dial through which a key reaches the winding arbor. A clock with one hole usually only keeps time, two means time and strike, and three adds a chime. More →