The ancient Romans used the hour to note a point of time, such as sunrise and sunset. They later added the hour of noon. At the beginning of the Christian era, the Romans divided the hours of daylight into five periods, which they marked on their sundials. In a.d. 605, the Christian church named the seven canonical hours, or hours of prayer. They were (1) matins (morning) and lauds (praise), (2) prime (first), (3) tierce (third), (4) sext (sixth), (5) nones (ninth), (6) vespers (evening), and (7) complin (complete). These hours marked only periods of daylight, beginning at 6 a.m. The nights were sometimes divided into watches, which marked the times when guards reported for duty or were changed. The length of the hour varied with the season. The winter hours were shorter than the summer hours, because there was less daylight during the winter.
By the 1500's, many churches and palaces in Europe had installed mechanical clocks with 12-hour dials. These clocks did not keep good time, and had to be set every sunshiny day at noon, when the sun was at its highest point, or on the meridian. From this we get the letters a.m., meaning ante (before) meridiem, or before noon. p.m. means post (after) meridiem, or after noon. When people first began to tell time by the clock, they substituted the word o'clock, meaning by the clock, for the word hour.
Confusion can result if the letters a.m. and p.m. are not used. European railroad and airplane timetables use a single 24-hour system. To avoid confusion, four figures are used. Thus, 1:00 a.m. is written 01:00. 1:00 p.m. is written as 13:00, and 12:00 midnight is 24:00. The United States armed forces also use this system, but without the colon. In conversation, all four figures are used, such as "O one hundred" for 1:00 a.m., "twelve hundred" for 12:00 noon, and "twenty-four hundred" for 12:00 midnight.
