What a complication is
In watchmaking, a complication is any function beyond the display of hours, minutes, and seconds. A date window is a complication. A chronograph is a complication. Even a power reserve indicator is a complication, because it requires its own mechanism reading the state of the mainspring.
Each one adds gears, jewels, and assembly time, which is why complication count tracks loosely with price. A simple three-hand movement is the easiest to build and service; a perpetual calendar is among the hardest. The question for a buyer is not how many complications a watch has, but which ones you will actually use.
The power reserve indicator
A power reserve indicator shows how much energy remains in the mainspring, usually as an arc or a linear gauge. It works by reading the angle of the mainspring barrel or a dedicated wheel that tracks the wind state. On an automatic watch it is a convenience — a reminder to wear or wind — and on a manual-wind watch it is closer to essential, since you are the only thing keeping the watch running.
A longer power reserve is not automatically better. A 41-hour reserve covers a weekend off the wrist; a 72-hour reserve covers a long weekend. Beyond that, the returns are marginal unless the watch sits unworn for long stretches. What matters more is that the reserve is honest — that the watch actually runs the hours it claims.
The date and day-date
The date complication advances a numbered wheel once per 24 hours, driven off the hour wheel through a slow or instant jump mechanism. An instant-date changes all at once near midnight; a slow-date crawls over several hours. The day-date adds a second wheel for the day of the week. Both are useful, but they cut a window into the dial, which is a design decision as much as a functional one.
A date adds a small amount of complexity and a few jewels; it is one of the most serviceable complications and the one most people actually use. If you never set the date, a no-date dial is cleaner and easier to live with.
The GMT and the travel watch
A GMT complication tracks a second time zone, usually with an additional hour hand that completes one revolution per 24 hours and a rotating 24-hour bezel. Set the bezel to the offset of your destination and the GMT hand reads home time against the 24-hour scale. It is the complication that grew out of aviation and remains genuinely useful for anyone who crosses time zones regularly.
A true GMT has a separately adjustable 12-hour hand, so you can reset local time without disturbing the home-time reference. A 'caller' GMT adjusts the 24-hour hand instead, which is less convenient in transit. The distinction is worth knowing if you travel.
The chronograph
A chronograph is a stopwatch built into a watch: independent pushers start, stop, and reset a seconds counter, usually with additional minute and hour totalizers. Mechanically it is demanding — it adds a clutch, a column wheel or cam, and several extra gears — which is why chronograph movements are thicker and more expensive to service.
A column-wheel chronograph switches functions with a toothed wheel; it is the traditional, higher-end construction and feels crisp at the pusher. A cam-lever chronograph uses a flat cam; it is simpler, cheaper, and perfectly functional. Most people never need the difference, but it is one of the places where mechanical refinement is most visible.
The moonphase
A moonphase tracks the lunar cycle with a disk printed with two moons, advanced through a 59-tooth gear that turns once per two lunar months — accurate to a day every two and a half years on the basic version. More precise moonphases use higher gear ratios that stay accurate for over a thousand years. It is the most poetic of the common complications: it does nothing practical, and it is beautiful for exactly that reason.
Complications are not a hierarchy of value. The right ones are the ones that match how you keep time.