Editing 12-hour clock (section)

==History and use==
[[File:Exeter Cathedral astronomical clock.jpg|thumb|upright|[[Exeter Cathedral Astronomical Clock]], showing the double-XII numbering scheme.]]
[[File:World War II RAF sector clock.jpg|thumb|World War II RAF [[sector clock]] that can be read either in 12- or 24-hour notation.]]

The natural day-and-night division of a calendar day forms the fundamental basis as to why each day is split into two cycles. Originally there were two cycles: 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 eventually evolved into the two 12-hour periods which are used today, one called "a.m." starting at midnight and another called "p.m." starting at noon.<ref name=EncyclopaediaBritannica/>

The 12-hour clock can be traced back as far as [[Mesopotamia]] and [[ancient Egypt]].<ref>{{Cite web|url=http://www.arcytech.org/java/clock/clock_history.html|archive-url=https://web.archive.org/web/20081013135550/http://www.arcytech.org/java/clock/clock_history.html|url-status=dead|archive-date=13 October 2008|title=The History of Clocks|date=13 October 2008|access-date=12 November 2017}}</ref> Both an Egyptian [[sundial]] for daytime use<ref>{{cite web|url=http://members.aon.at/sundials/berlin-egypte.htm|title=Berlin instruments of the old Eg.time of day destination|website=members.aon.at|access-date=13 June 2006|archive-url=https://web.archive.org/web/20061115233642/http://members.aon.at/sundials/berlin-egypte.htm|archive-date=15 November 2006|url-status=live}}</ref> and an Egyptian [[water clock]] for night-time use were found in the tomb of Pharaoh [[Amenhotep I]].<ref>{{cite web|url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks|work=A Walk Through Time|title=Early Clocks|date=12 August 2009 |publisher=[[National Institute of Standards and Technology]]}}</ref> Dating to {{Circa|1500 BC}}, these clocks divided their respective times of use into 12&nbsp;hours each.

The [[ancient Rome|ancient Romans]] also [[Roman timekeeping|used a 12-hour clock]]: daylight and nighttime were each divided into 12&nbsp;equal intervals (of varying duration according to the season).<ref>{{Cite magazine |last= Bartels |first=Meghan |title=Early Tech Adopters in Ancient Rome Had Portable Sundials |url=https://www.smithsonianmag.com/innovation/early-tech-adopters-ancient-rome-had-portable-sundials-180962225/ |access-date=2025-03-08 |magazine=Smithsonian Magazine |date=February 20, 2017}}</ref> The nighttime hours were grouped into four ''[[Watchkeeping|watches]]'' (''vigiliae'').<ref>{{cite web |url=https://www.thehourglass.com/story/timekeeping-in-the-roman-army |title=Timekeeping In the Roman army |date=7 July 2018}}</ref>

The first mechanical clocks in the 14th&nbsp;century, if they had dials at all, showed all 24&nbsp;hours using the [[24-hour analog dial]], influenced by astronomers' familiarity with the [[astrolabe]] and sundial and by their desire to model the [[Earth's rotation|Earth's apparent motion around 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 Cathedral clock|Wells]] and [[Exeter Cathedral#Clock|Exeter]].

Elsewhere in Europe, numbering was more likely to be based on the 24-hour system (I to XXIV). The 12-hour clock was used throughout the [[British Empire]].

During the 15th and 16th&nbsp;centuries, the 12-hour analog dial and time system gradually became established as standard throughout Northern Europe for general public use. The 24-hour analog dial was reserved for more specialized applications, such as [[astronomical clock]]s and chronometers.

Most analog clocks and watches today use the 12-hour dial, on which the shorter hour hand rotates once every 12&nbsp;hours and twice in one day. Some analog clock dials have an inner ring of numbers along with the standard 1-to-12 numbered ring. The number 12 is paired either with a 00 or a 24, while the numbers 1 through 11 are paired with the numbers 13 through 23, respectively. This modification allows the clock to also be read in [[24-hour notation]]. This kind of 12-hour clock can be found in countries where the 24-hour clock is preferred.

===Use by country===
{{Main|Date and time representation by country}}
[[File:Westclox Big Ben.jpg|thumb|right|Typical [[Clock#Analog|analogue 12-hour clock]]]]
[[File:12 24 Hours World Map.svg|thumb|World map showing the usage of 12 or 24-hour clock in different countries{{legend|#f0027f|24-hour}}{{legend|#990052|24-hour (12-hour orally)}}{{legend|#7fc97f|Both in common use}}{{legend|#386cb0|12-hour (except in special circumstances such as [[aviation]])}}]]
In several countries the 12-hour clock is the dominant written and spoken system of time, predominantly in nations that were part of the former British Empire, for example, the [[United Kingdom]], [[Republic of Ireland]], the [[United States]], [[Canada]] ([[Date and time notation in Canada#Time notation in French|excluding Quebec]]), [[Australia]], [[New Zealand]], [[South Africa]], [[India]], [[Pakistan]], and [[Bangladesh]], and others follow this convention as well, such as [[Mexico]] and the former American colony of the [[Philippines]]. Even in those countries where the 12-hour clock is predominant, there are frequently contexts (such as science, medicine, the military or transport) in which the 24-hour clock is preferred. In most countries, however, the [[24-hour clock]] is the standard system used, especially in writing. Some nations in Europe and Latin America use a combination of the two, preferring the 12-hour system in colloquial speech but using the 24-hour system in written form and in formal contexts.

The 12-hour clock in speech often uses phrases such as'' ...&nbsp;in the morning'','' ...&nbsp;in the afternoon'','' ...&nbsp;in the evening'', and ''...&nbsp;at night''. ''Rider's British Merlin'' almanac for 1795 and a similar almanac for 1773 published in London used them.<ref>{{cite book|url=http://catalogue.nla.gov.au/Record/4881611|title=National Library of Australia catalogue entry for ''Rider's British merlin: for the year of Our Lord God 1795''|year=1795 |publisher=Printed for the Company of Stationers, and sold by R. Horsfield, at Stationers-hall, near Ludgate-street |archive-url=https://web.archive.org/web/20130522172644/http://catalogue.nla.gov.au/Record/4881611|archive-date=22 May 2013|url-status=live}}</ref> Other than in English-speaking countries and some Spanish-speaking countries, the terms ''a.m.'' and ''p.m.'' are seldom used and often unknown.{{efn-lg|name=Other}}

===Computer support===
In most countries, computers by default show the time in 24-hour notation. Most operating systems, including [[Microsoft Windows]] and [[Unix-like]] systems such as [[Linux]] and [[macOS]], activate the 12-hour notation by default for a limited number of language and region settings. This behaviour can be changed by the user, such as with the [[Microsoft Windows|Windows]] operating system's "Region and Language" settings.<ref>{{cite web|url=http://www.bleepingcomputer.com/tutorials/customize-how-time-is-displayed-in-windows/|title=How to customize how the time is displayed in Windows|author=Lawrence Abrams|date=13 December 2012|access-date=26 October 2013|publisher=Bleeping Computer|archive-url=https://web.archive.org/web/20131029183705/http://www.bleepingcomputer.com/tutorials/customize-how-time-is-displayed-in-windows/|archive-date=29 October 2013|url-status=live}}</ref>