12-hour clock
The 12-hour clock is a time-keeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (am, Latin for "before noon") and post meridiem (pm, Latin for "after noon"). Each period consists of 12 hours numbered 12, 1, 2, ..., 11.
Because the numbering of hours starts at 12, 12:00 am is properly the midnight at the start of the day, and 12:00 pm is noon. However, the ordering of the hours means that many people, even many of those who use the 12-hour clock regularly, are confused as to which of 12:00 am and 12:00 pm is which. Therefore, for clarity, it is often best to write "noon" instead of "12:00 pm". Using this clock, there is no unambigious way to write "12:00 am", as "midnight" can mean either the midnight at the start of the day or the midnight at the end.
Historically the 12-hour clock originated with the Sumerians, although their clock varied seasonally, as it had 12 hours from dusk to dawn, and 12 hours from dawn to dusk.
See also
