<P> During the 15th and 16th 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 clocks and chronometers . </P> <P> Most analog clocks and watches today use the 12 - hour dial, on which the shorter hour hand rotates once every 12 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 be read also in the 24 - hour notation . This kind of 12 - hour clock can be found in countries where the 24 - hour clock is preferred . </P> <P> 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 (the United Kingdom, the United States, Australia, India, Pakistan, Malaysia, Bangladesh etc .), though a few others (Mexico, Egypt, and the former American colony of the Philippines) follow this convention as well . In most countries, however, the 24 - hour clock is the standard system used, especially in writing . Some nations 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 . This is true in some European nations and Latin America . </P> <P> The 12 - hour clock in speech often uses phrases such as in the morning, in the afternoon, in the evening, and at night . Outside of English - speaking countries, the terms a.m. and p.m. are seldom used and often unknown . In England, the descriptive phrases were universal until relatively recently; e.g., Rider's British Merlin almanac for 1795 (published in London) uses them, and so does a similar almanac for 1773 . </P>

When did am and pm start being used