Nautical time units are a system of measuring time used by sailors and navigators. The most common unit is BELLS, which is a five-letter word that is used to divide the four-hour watch into eight half-hour periods. Each period is marked by the ringing of a bell, with one bell for the first half-hour, two BELLS for the second half-hour, and so on. This system is still used today by mariners and is an important part of maritime culture.