Kilometres per hour
The kilometre per hour (American English: kilometer per hour) is a unit of speed, expressing the number of kilometres travelled in one hour.
The unit symbol (not abbreviation) is km/h. Worldwide, it is the most commonly used unit of speed on road signs and car speedometers.
Although the metre was formally defined in 1799, the term "kilometres per hour" did not come into immediate use – the myriametre (10,000 metres) and myriametre per hour were preferred to kilometres and kilometres per hour. In 1802 the term "myriamètres par heure" had appeared in French literature and many French maps printed in the first half of the nineteenth century had scales in leagues and myriametres, but not in kilometres. The Dutch on the other hand adopted the kilometre in 1817 but gave it the local name of the mijl.
Several representations of "kilometres per hour" have been used since the term was introduced and many are still in use today; for example, dictionaries list "km/h", "kmph" and "km/hr" as English abbreviations. The SI representations, classified as symbols, are "km/h", "km h−1" and "km·h−1".
The use of abbreviations dates back to antiquity, but abbreviations for "kilometres per hour" did not appear in the English language until the late nineteenth century.
The kilometre, a unit of length, first appeared in English in 1810, and the compound unit of speed "kilometers per hour" was in use in the US by 1866. "Kilometres per hour" did not begin to be abbreviated in print until many years later, with several different abbreviations existing near-contemporaneously.
With no central authority to dictate the rules for abbreviations, various publishing houses have their own rules that dictate whether to use upper-case letters, lower-case letters, periods and so on, reflecting both changes in fashion and the image of the publishing house concerned. For example, news organisations such as Reuters and The Economist require "kph".
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