A complication on a watch is any function other than telling the time. They can range from simple and pretty common to very rare and highly complex that take years to create.
The simplest complication is the date display. There are four common varieties of date displays
Date Window – The window is also referred to as an aperture. Commonly found at the 3 o’clock, 4, 4:30, or 6 o’clock position.
Big Date – This display allows a much bigger view of the date and is much more legible than traditional date window. Sometimes has two windows with the left on showing 0-3 and the right window displaying 0-9.
Pointer Date – A center hand with an arrow or crescent pointing to the date along the outside of the dial. Sometimes called a “Bankers” date.
Subsidiary Dial – Displays the date on a small sub-dial and is often used with other complications.
Other varieties of date displays include:
Day-Date – The Day-Date adds the day of the week to the date complication. It is typically set using the crown; moving it in one direction changes the day, while the other direction changes the date. It may also be set using a small indented button on the side of the case with a special tool called a stylus.
Equation of Time – An Equation of Time (EOT) perpetual calendar is the absolute pinnacle of calendar watches. It incorporates all the features of a standard perpetual calendar with one additional feature: the measurement, in minutes, of the difference between our “calendar time” and the actual “solar time.”
Following date complications, the chronograph is the most common complication: a watch that has a stopwatch built into the movement. Types of chronographs include:
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.
Tachymeter – A Tachymeter is an instrument for measuring speed that is commonly found on watches. Typically, a scale is placed on the outer or inner bezel of a watch and is generally only found in conjunction with chronographs. A Tachymeter measures units per hour, generally miles or kilometers. In order for a tachymeter to work, you must move at a fixed rate of speed and distance (e.g., 1 mile or 1 kilometer).
Dual Time Zone (Travel Complications)
Dual Time Zone complications help determine the time in another time zone; they include:
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, which completely changes the functionality of the watch.
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.
Tourbillon, Moonphase, and other Complications
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 in mechanical watches.
Tourbillon – Invented by A.L. Breguet, the Tourbillon improves the balance of the watch, eliminating timekeeping errors caused by gravity and changing watch positions. Though not strictly necessary for accuracy purposes today, it 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.