Seamaster Diver 300M 41 mm, steel on steel

Label given to a watch which has undergone precision tests and received a certificate from an official body (COSC).

The day of the month, displayed in a window on a watch dial at the 3 or 6 o’clock position, or within a subdial at 9 o'clock.

A GMT function means that the watch has a second hour hand which completes one full rotation in 24 hours and a 24-hour index on the dial. Particularly useful for wearers who wish to track two time zones.

Decompression system which allows helium to escape from inside the watch when the watch is worn for professional use in pressure chambers. (Long-term underwater work, crude oil exploration, etc.)

Self-locking crown, screwed into the tube of the case, used for highly water resistant divers' watches.

A bezel which turns in only one direction. Particularly useful for divers who cannot accidentally rotate it in the wrong direction, which could cause them to miscalculate their dive times. A rotating bezel makes it easy to measure elapsed time.