A complication is any function on a watch, other than the display of the time. Complications can range from simple displays of the date, to extremely rare works of high horology that combine numerous functions.
Date: The simplest complication on a watch is the date display.
Date Window: This window is also referred to as an aperture. On some watches the color of the numbers alternate between red and black, this is called "Casino" date display.
Big Date: This display allows a much larger view of the date and is significantly more legible than the traditional date window. The variation sometimes has two windows, the left one displaying 0-3, and the right window displaying 0-9.
Date Wheel: A center hand with an arrow, or crescent, pointing to the date along the outside periphery, also known as a chapter of the dial. This is sometimes called a 'Bankers' date.
Subsidiary Dial: Displays the date on a small sub-dial. Most often used in conjunction with other complications.
Classic Day-Date Dial
Side by Side Day-Date Dial
Triple Calendar: Also called a 'Complete Calendar', this calendar is a further elaboration of date display, adding the month of the year.
Triple Calendar w/Aperture
Windows and Date Wheel
Perpetual Calendar: The Perpetual Calendar is the most complex type of calendar feature that exists on a watch. It accurately displays the date, day, month, and year, and even takes into account the leap year. It will need correction in 2100, however, when the leap year will be ignored.
Annual Calendar Complication: An Annual Calendar, an intermediary complication, is between a perpetual calendar and a triple calendar. It does not take leap years into account; therefore it will continue to run to the 31st in February, before advancing to the first of March.
A chronograph watch has a stopwatch built into the movement.Types of Chronographs:
Mono-Poussier (One-Button Chronograph): Originally, all chronographs were Mono-Poussiers; Breitling introduced the two-button chronograph in 1923. The difference between a one and two-button chronograph is that the one-button model cannot measure interrupted time spans.
Retour-en-Vol (Flyback Chronograph): The Flyback chronograph is specially engineered so that when a second button is pushed, while the chronograph is running, all the counters reset and immediately start again from zero. This feature was originally designed for pilots where split second accuracy is necessary for precise navigation. Flybacks are the exception, not the rule.
Rattrapante (Split-Second Chronograph): It's easy to tell if a chronograph watch is a Rattrapante; it will have three pushers on the case and two second-hands on the chronograph, one on top of the other.
Dual Time Zone (Travel) Complications
This family of complications helps determine the time in other time zones.
Dual Movement: While not technically a complication, the Dual Movement is a watch that contains two separate movements, each running from its own power source and each being set independently.
Dual Time: For Dual Time watches, both displays are powered by the same movement.
GMT (Greenwich Mean Time): Your watch displays two or more time zones.
GMT (Greenwich Mean Time) with Independent Hour Hand: This variety of GMT is a further development of the original. What makes it different is that the regular hour hand is set independently of the 24 hour hand. This changes the functionality of the watch completely.
GMT (Greenwich Mean Time) with Fixed Hour Hand: Introduced by Rolex in the 1950's, this GMT complication is considered a pilot's watch. Its unique additional hour hand makes one revolution around the dial per day; pointing to twelve indicates midnight and pointing to six indicates noon.
World Time Zone: The World Time Zone feature has a rotating inner bezel with 24-hour display, part of the watch movement, and an outer bezel, listing the major cities in each of the 24 time zones. The outer bezel is set by the user. The inner bezel, marked to 24, makes one complete revolution per day.
Tachymeter, Tourbillon and Other Complications
Tachymeter (Tachometer) Complication: The Tachymeter feature can be used to measure the speed at which the wearer moves, over a given distance. Typically placed on the bezel and generally only found with chronographs, the Tachymeter measures units per hour.
The Tourbillon: Invented by A.L. Breguet, the Tourbillon improves the balance of the watch, eliminating only timekeeping errors gravity and changing watch position cause. Though not strictly necessary for accuracy purposes today, is commonly appreciated as a feature of high-quality watches. The Tourbillon is extremely rare and requires an enormous amount of time and skill to construct.
Moonphase Complication: A traditional and aesthetically pleasing feature, the Moonphase complication shows if it is a full, half, quarter, or new moon. Originally, it was primarily used by sailors to gauge tides.
Power Reserve Indicator: The Power Reserve Indicator measures the amount of power remaining in the watch, by the tension of the mainspring and displays. Some watches have a power reserve of up to 10 days, in which the indicator displays days, not hours. This useful complication is found exclusively on mechanical watches.
Jump Hour: The Jump Hour, a complication displaying the hour in an aperture, instantly changes every 60 minutes.
Minute Repeater: A Minute Repeater is a movement that chimes out time when a lever on the side of the case is activated. It was a fairly common complication for pocket watches around the 18th and 19th centuries. It is now produced as a collectable, rather than a tool.