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Watchstanding, or watchkeeping, in nautical terms concerns the division of qualified personnel to operate a ship continuously around the clock. On a typical sea going vessel, be it naval or merchant, personnel keep watch on the bridge and over the running machinery. The generic bridge watchstanders are a lookout and an officer or mate who is responsible for the safe navigation of the ship. Safe navigation means keeping the vessel on track and away from dangers as well as collision avoidance from other shipping. An engineering specialist ensures that running machinery continues to operate within tolerances and depending on the vessel, this can also be accomplished from the bridge. A secondary function of watchkeeping is the ability to respond to emergencies, be it on own ship or involving other ocean users.

Watch durations will vary between vessels due to a number of reasons and restrictions. The traditional three-watch system from the days of sail where the ships company was divided into three and the day divided into six watches of four-hours duration, such that an individual would keep two four-hour watches each day separated by an eight-hour time for sleep or recreation. Examples of other systems include 4 by 6-hour and 7's and 5's when a warship has half its ships company on watch at a time.

The watches kept on wind ships - the square-rigged barques or windjammers of the late 19th century and in the British Royal Navy - consisted of 5 four-hour periods and 2 two-hour periods in the following pattern -
0000 - 0400 night watch
0400 - 0800 morning watch
0800 - 1200 forenoon watch
1200 - 1600 afternoon watch
1600 - 1800 first dogwatch
1800 - 2000 second dogwatch
2000 - 2400 evening watch
This pattern allowed the two watches, known as the 'port' and 'starboard' watches, to alternate from day to day, so that the port watch had the night watch one night and the starboard watch had it the next night.

Traditionally the changes of the watches are marked by ringing the ship's bell. The bell is sounded after each half hour, 1 'bell' at the first half hour, then 2 'bells, up to 4 or 8 'bells' to mark the end of the watch and the start of the next. So you have to qualify '6 bells' or '4 bells' by the name of the watch it occurs in. For example '6 bells of the morning watch' means 0700h and '4 bells of the second dog-watch' means 2000h and so on. 5 through 8 'bells' of the dog-watch do not exist. 0400 is '8 bells of the night watch'.