# kilometer per hour

Kilometer (Brazilian Portuguese) or kilometer (European Portuguese) per hour (km/h) is a physical unit or unit of speed, expressing the number of kilometers traveled in one hour.

## History

Although the meter was formally defined in 1799, the term "kilometers per hour" did not come into immediate use - the miriametre (10,000 meters) and miriametre per hour were preferred to kilometers and kilometers per hour. In 1802, the term "miriamètres par heure" appeared in French literature, and many French maps printed in the first half of the 19th century had scales in leagues and miriametres, but not kilometers. The Dutch, on the other hand, adopted the kilometer in 1817, but gave it the local name of the mijl (Dutch mile).
Internationally, km/h is the most commonly used unit of speed on road signs and road vehicle speedometers.

## Notation history

The SI representations, classified as symbols, are "km/h", " km h−1 " and " km · h−1". Various other representations of "kilometers 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" in English literature.

## Abbreviations

Abbreviations for "kilometers per hour" did not appear in the English language until the late 19th century.
The kilometer, a unit of length, first appeared in English in 1810, and the composite unit of speed "kilometers per hour" was in use in the US by 1866 "kilometers per hour" did not begin to be abbreviated in print until many years. later, with several different abbreviations existing almost contemporaneously.
With no central authority to dictate the rules for abbreviations, many publishers have their own rules governing the use of capital letters, lowercase letters, periods and so on, reflecting changes in fashion and the image of the concerned publisher. For example, news organizations like Reuters and The Economist require "kph".

## Unit symbols

In 1879, four years after the signing of the Metro Treaty, the CIPM proposed a range of symbols for the various metric units then under the auspices of the CGPM. Among them was the use of the symbol "km" for "kilometer". In 1948, as part of its preparatory work for the SI, the CGPM adopted symbols for many units of measurement that did not have universally accepted symbols, one of which was the symbol "h" for "hours". At the same time, the CGPM formalized the rules for combining units - the quotients can be written in one of three formats, resulting in "km/h" , "km h−1 " and "km · h−1 " being representations valid "kilometers per hour" The SI standards, which were MKS-based rather than CGS-based, were published in 1960 and have since been adopted by many authorities around the world, including academic publishers and legal authorities.
The SI explicitly states that unit symbols are not abbreviations and must be written using a very specific set of rules. M. Danloux-Dumesnils provides the following justification for this distinction: It has already been said that, according to Maxwell, when we write down the result of a measurement, the numerical value multiplies the unit. Therefore, the unit name can be replaced by a sort of algebraic symbol, which is shorter and easier to use in formulas. This symbol is not just an abbreviation, but a symbol that, like chemical symbols, must be used in a precise and prescribed manner.SI and therefore the use of "km/h" (or "km h−1" or " km · h−1" ) has now been adopted worldwide in many areas related to health and safety and in metrology beyond the SI unit meters per second ("m/s", "ms−1" or "m · s− 1" ). SI is also the preferred measurement system in academia and education.
Alternate abbreviations