complication – any “function” added to a watch, such as a minute repeater, countdown timer, stopwatch, altimeter, asthometer, pulsometer, calendar, moon phase indicator, split-second chronograph, power reserve indicator, alarm, etc.

simple calendar – is a complication that shows the date of the month. A day/date shows the date of the month and the day of the week. A complete calendar shows the day, date, and the month or moon phase.

annual calendar – displays the date, day of the week and month with only one manual adjustment required per year for February (except during a leap year).

perpetual calendar – an elaborate complication that keeps track of the day, month, date, and sometimes even the moon phase, zodiac signs, decade, century, and which adjusts for the length of the month and for leap years. (Accurate until 2100)

moon phase – displays new moon, first quarter moon, full moon, and last quarter moon by means of a disk that rotates beneath a small aperture. The 29 and 1/2 day cycle of the moon.

deadbeat seconds – A hand that jumps when the second has elapsed. Generally, the jump of a hand is a distinctive feature of a quartz watch. In mechanical watchmaking, this is a function.

dual-time zone – measures local time as well as time in another time zone. Often also called, Greenwich Mean Time (GMT), a watch that has the capability of displaying two different time zones.

foudroyante – small dial that is marked 0-8. The hand on the dial completes a sweep every second which is an elapsed time of 1/8th of a second for each number.

depth-alarm – a complication on a diver’s watch that sounds an alarm when the wearer exceeds pre-set depth. The alarm stops when the diver ascends above pre-set depth.

flyback – a function that allows a chronograph to be reset to zero without having to stop the chronograph first.

jump hour – a display in which the hour, shown through an aperture, instantly changes every 60 minutes.

split-seconds chronograph – or “rattrapante” or “doppelchrono” has two seconds hands, the first push starts both hands together, the second push stops one hand while the other continues, and another push allows the stopped hand to catch up with the moving seconds.

worldtimer – a timepiece that displays the current time in any part of the world, from 24 up to 37 time zones.

yacht countdown timer – a countdown timer that sounds warning signals during the countdown to a boat race.

minute repeater – a highly complicated function that can strike the time in hours, quarters, or minutes by means of a push piece. Before the light bulb, minute repeaters allowed their owner to listen to the time without the need for light. Strikes/Chimes the hour, half hour, minutes, seconds, or some combination of the two.

tourbillon – (also called “whirlwind”) eliminates errors of rate due to earth’s gravity of vertical positions. A complex mechanism requiring the highest watchmaking skills: consists of a mobile carriage carrying all the parts of the escapement. It is a regulating device which corrects for the gravity-caused differences in runtime in a mechanical watch.

power reserve indicator – a subdial used to show how much power remains before the watch stops.

tidegraph – tides are the periodic rise and fall of the water of oceans, seas, bays and other bodies of water caused mainly by the gravitational interactions between the Earth, Moon and Sun. Tides rise and fall about every six hours. The tide graph indicates tidal movement based on the Moon’s transit over the meridian and the lunitidal interval.

equation of time – The equation of time is the difference between true solar time and mean time. True solar time, given by sundials, varies from day to day because of the Earth’s elliptical orbit, and according to the longitude of the point of observation. Mean time, given by watches, ignores these variations and for every day of the year mathematically divides time into equal hours. Apparent solar time, can be ahead (fast) by as much as 16 minutes 33 seconds (around 3 November), or behind (slow) by as much as 14 minutes 6 seconds (around 12 February).

chronograph – stopwatch function that uses subdials to keep track of second, minutes, and hours.

retrograde – an hour, minute, seconds or calendar hand which moves across a scale and, at the end of its cycle, returns immediately to zero to begin again (not to be confused with “regulator dial”).

slide rule bezel – a rotating bezel that is printed with a logarithmic scale and assorted other scales and is used in conjunction with fixed rules of mathematics to perform general mathematical calculations or navigational computations.

tachymeter – instrument for measuring speed over a measured distance. A racing car covers 1 mile in 30 seconds. The sweep second hand, when stopped at the end of the mile, would point to the 120 on the tachometer. The average speed in 120 mph.

alarm – a function that can be used to set an audible (typically buzz sound) alarm.

dive bezel – a graduated bezel that measures 0-60 and rotates in only one direction.

telemeter – stopwatch or chronograph function with a scale that measures the distance of something from the wearer of the watch through the amount of time it takes for sound to travel.