The 12 hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (a.m., from Latin, literally "before the middle of the day", idiomatically "approaching midday") and post meridiem (p.m., "past midday"). Each period consists of 12 hours numbered 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Time periods run 12:00 through 11:59 either ante meridiem or post meridiem. When referencing twelve o'clock the descriptor is either noon or midnight, without the ambiguity.
The 12-hour clock originated in Egypt. However, the lengths of their hours varied seasonally, always with 12 hours from sunrise to sunset and 12 hours from sunset to sunrise, the hour beginning and ending each half-day (four hours each day) being a twilight hour. An Egyptian sundial for daylight use and an Egyptian water clock for nighttime use found in the tomb of Pharaoh Amenhotep I, both dating to c. 1500 BC, divided these periods into 12 hours each.
The Romans also used a 12-hour clock: the day was divided into 12 equal hours (of, thus, varying length throughout the year) and the night was divided into three watches. The Romans numbered the morning hours originally in reverse. For example, "3 a.m." or "3 hours ante meridiem" meant "three hours before noon", compared to the modern meaning of "three hours after midnight".
The first mechanical clocks in the 14th century, if they had dials at all, showed all 24 hours, using the 24 hour analog dial, influenced by astronomers' familiarity with the astrolabe and sundial, and their desire to model the apparent motion of the sun. In Northern Europe, these dials generally used the 12 hour numbering scheme in Roman numerals, but showed both a.m. and p.m. periods in sequence. This is known as the Double-XII system, and can be seen on many surviving clock faces, such as those at Wells and Exeter. Elsewhere in Europe, particularly in Italy, numbering was more likely to be based on the 24 hour system (I to XXIV), reflecting the Italian style of counting the hours.
During the 15th and 16th centuries, the 12 hour analog dial and time system, with its simpler and more economical construction, gradually became established as standard throughout Northern Europe for general public use, with the 24 hour analog dial reserved for the more specialist applications, such as for astronomical clocks and chronometers.
Today, most analog clocks and watches use the 12-hour dial, where the hour hand (shorter and sometimes thicker) commonly rotates once over the course of every 12 hours, and twice in a day. These are used even in cultures where the 24-hour notation is otherwise preferred. Some 12-hour dials show the numbers 13 to 23 written inside the primary 1 to 12 ring. Wikipedia
