From the early days of the navy until today timekeeping on naval ships has been carried out by ringing a bell at regular intervals. The system of watches was designed to operate in conjunction with the use of a half-hour sand clock. Each time the clock was turned a bell was rung: 'one bell' was rung half an hour into the watch, 'two bells' an hour into it, and so on.  The bell was rung in groups of two chimes, so eight bells would sound as 'ding-ding, ding-ding, ding-ding, ding-ding'.

The sailors on board a naval vessel were divided into two watches, and would alternate duties when the last bell was rung. Most watches lasted for a four-hour period, so when 8 bells were rung the watch was ended. There were five normal watches, which ran between eight pm on one day and four pm on the following day.

Between four pm and eight pm there were two 'dog watches', each lasting two hours. The effect of this was to cause the daily schedule of watches to alternate so that the group of sailors keeping first watch on one day would not have to keep it again the next day. The Dog watches also allowed the sailors from both watches to have a break in order to eat their evening meal. The end of the first dog watch was marked by four bells, and the end of the second by eight.

Only one bell is sounded at 6.30, during the second dog watch, when the system would seem to call for five. According to naval tradition this stems from the mutinies of 1797, when in one port five bells in the dog watch was the prearranged signal for the rebellion to begin. The port authorities got wind of this, however, and averted the mutiny by causing only one bell to be rung at 6.30.

Viewers of nautical dramas will be familiar with the cry 'eight bells and all's well', which simply signified that the watch had passed without incident. The ringing of naval bells has given rise to a number of other familiar expressions, including the phrase 'to knock seven bells' out of someone - to beat someone up. The sense of this is a little obscure, although the probable meaning is that by knocking seven bells out of a person you almost kill them; if you knocked eight bells out of them, the result would presumably be fatal.

Another phrase arising out of the bell timekeeping system is 'to warm the bell'. This means to do something before the proper time, for example leaving work early. Again, the precise origin of this phrase is obscure, although it may be an analogy with warming a clock, which causes its mechanism to run faster.