twelvehour

Twelvehour refers to the 12-hour timekeeping system in which the day is divided into two cycles of twelve hours. In this system, hours are numbered from 1 to 12, and times are typically followed by indicators to distinguish the two halves of the day, most commonly AM for ante meridiem and PM for post meridiem.

In practice, the 12-hour clock coexists with the 24-hour clock in many regions. The 12-hour format is common in everyday life in several countries, particularly in North America, where times are often spoken and written with AM or PM. The 24-hour format is frequently used in official, military, or logistical contexts to avoid ambiguity.

The concept has historical roots in ancient civilizations. The division of the day into twelve parts likely arose from the use of duodecimal counting and from the sun’s apparent movement across the sky. Early timekeeping devices such as sundials produced variable-length hours, and medieval Europe adopted a 12-hour dial on mechanical clocks. Today, each hour in a 12-hour cycle is a fixed 60 minutes, but the division of daylight and night into equal twelve-hour segments is a cultural convention.

In contemporary contexts, the term twelvehour is used in discussions of time formats, clock interfaces, or software to denote the 12-hour representation as distinct from the 24-hour format. See also 24-hour clock and timekeeping.