What it does
A minute repeater chimes the current time when activated. The standard pattern: a low tone for each hour (1-12 strikes), a two-tone double strike for each quarter-hour past the hour (0-3 strikes), and a high tone for each minute past the last quarter (0-14 strikes). At 7:38, the watch would chime 7 low tones (7 hours) + 2 double tones (30 minutes / 2 quarters) + 8 high tones (8 minutes past the half hour). The chime takes 10-30 seconds to complete.
How it works
The mechanism uses two small steel gongs (essentially miniature tuning forks) wrapped around the inside of the movement, struck by tiny hammers. The repeater is activated by sliding a lever on the case-band, which winds a small dedicated spring; the spring drives a snail-shaped cam that times the strikes. The hammers strike a different gong for high vs low tones; both gongs simultaneously for the quarter-hour double strike.
Why it exists
Pocket-watch minute repeaters originated in the 1750s as a practical way to tell the time in dim or dark environments before electric light. By the 19th century they had become a high-luxury status complication. The modern wristwatch minute repeater (which started appearing in the 1900s and became regular in the 1990s revival) is purely decorative; you can read a watch in the dark with the lume.
What makes a great repeater
Sound quality is everything. Two factors dominate: case material (steel and titanium ring better than gold/platinum; precious metals damp the sound) and movement architecture (Patek and AP repeaters use specific gong-tuning processes that take days per piece). The very best minute repeaters (Patek 5074P, Audemars Piguet Code 11.59 Supersonnerie, Vacheron Patrimony minute repeater) are tuned by hand by single watchmakers; production volumes are 5-20 pieces per year. Retail starts at CHF 200,000 and runs to seven figures.