The primary function of a watch is to tell us the time, and the most basic type of watch counts the passing of hours, minutes, and usually seconds (a time-only watch). But pocket and wristwatches also can have a variety of added features:
24 hour time: This type of watch not only tells the time on a 12-hour dial but also indicates the time in “24-hour” format. Notable models include the Hamilton 4992b (exclusively 24 hours), and the Rolex GMT (both 12 and 24 hours, can be set for two time zones).
Calendar: A calendar Watch has many variations, including the day of the month, the day of the month and day of the week, the day of the month/week, and the phase of the moon. A standard calendar watch will need correction at the end of each month for the number of days in the month. Engineers designed a highly complex variation called the perpetual calendar (not to be confused with the Rolex Perpetual, actually Rolex’s name for their automatic). A notable example is the ‘Patek Philippe Perpetual calendar chronograph’ introduced initially around 1941.
Automatic: The automatic watch utilizes movement from your arm and body to keep the watch wound up. Other names include ‘self-winding’ watch and Rolex’s Perpetual (not to be confused with a perpetual calendar watch, see above).
Chronograph: These have a “stopwatch” function. On either side of the stem, there usually are buttons that operate the start/stop and reset function. Subdials include a 30 or 45 minute counter and a 12 or 24-hour timer.
Repeater: Designers developed the repeater complication out of the need to tell the time in the dark. Sliding a lever will prompt the watch to ‘strike’ the hour and quarter-hour increments (quarter repeater), or in 5-minute increments (5-minute repeater).
Chronometer: Watches such as Omega and Rolex come with the designation “Certified Chronometer,” or “Officially Certified Chronometer.” Modern watches that carry this designation are certified by the “Contrôle Officiel Suisse des Chronomètres” (COSC). These watches are held to a very high standard of accuracy. Various methods and governing agencies have certified antique and vintage watches in times past. Note also that many makers printed the term “Chronometer” on dials of lesser quality watches that are not certified.