12-hour clock

The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods:a.m. (from the Latin ante meridiem, meaning "before midday") and p.m. (post meridiem, "after midday"). Each period consists of 12 hours numbered: 12 (acting as zero),1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 24 hour/day cycle starts at 12 midnight (often indicated as 12 a.m.), runs through 12 noon (often indicated as 12 p.m.), and continues to the midnight at the end of the day.

The 12-hour clock was developed over time from the mid-second millennium BC to the 16th century AD.

History and use

The natural day/night division of a calendar day forms the fundamental basis as to why each day is split into two cycles. Originally these were one cycle which could be tracked by the position of the Sun (day) followed by one cycle which could be tracked by the Moon and stars (night). This would eventually evolve into the two 12-hour periods that started at midnight (a.m.) and noon (p.m.) which are used today. Noon itself is rarely abbreviated today, but if it is, it is denoted M.