Ship's bell Clocks provide an audible signal for the time on board a ship by the striking of a bell. As each "watch" begins, one strike of the bell indicates the passing of the first half-hour. Two strikes represents the passing of the second half-hour. If a sailor's watch duty begins at 8:00 a.m. (Forenoon Watch), and he hears the bell strike three times, he will immediately know that it is 9:30 without having to see a clock. This sequence continues until the sailor's four-hour watch is complete and the bell is struck eight times.
The "watches" or periods of duty on board break down as follows:
| Midnight to 4 a.m. | Middle Watch | |
| 4 a.m. to 8 a.m. | Morning Watch | |
| 8 a.m. to Noon | Forenoon Watch | |
| Noon to 4 p.m. | Afternoon Watch | |
| 4 p.m. to 6 p.m. | First Dog Watch | |
| 6 p.m. to 8 p.m. | Last Dog Watch | |
| 8 p.m. to Midnight | First Watch |
Note that each watch consists of four hours except the Dog Watches. Therefore a sailor on the First Dog Watch will only hear four bells at the conclusion of his two hour watch duty.
In domestic or civilian use, ships bell clocks never stray from the four-hour or eight strike bell sequence. As opposed to the nautical day of seven periods, ship's bell clocks used domestically divide the day into six equal parts.