Kilometres Per Hour - Notation History - Kilometers Per Hour As A Symbol

Kilometers Per Hour As A Symbol

The use of symbols to replace words dates back to at least the late Mediaval era when Johannes Widman, writing in German in 1486, used the symbols "+" and "-" to represent "addition" and "subtraction". In the early 1800s Berzelius introduced a symbolic notation for the chemical elements derived from the elements' Latin names. Typically, "Na" was used for the element sodium (Latin: natrium) and H₂O for water.

In 1879, four years after the signing of the Treaty of the Metre, the CIPM proposed a range of symbols for the various metric units then under the auspices of the CGPM. Among these were the use of the symbol "km" for "kilometre".

In 1948, as part of its the preparatory work for the SI, the CGPM adopted symbols for many units of measure that did not have universally-agreed symbols, one of which was the symbol "h" for "hours". At the same time the CGPM formalised the rules for combining units - quotients could be written in one of three formats resulting in "km/h", "km h-1" and "km·h-1" being valid representations of "kilometres per hour". The SI standards, which were MKS-based rather than CGS-based were published in 1960 and have since then have been adopted by many authorities around the globe including academic publishers and legal authorities.

The SI explicitly states that unit symbols are not abbreviations and are to be written using a very specific set of rules. M. Danloux-Dumesnils provides the following justification for this distinction:

It has already been stated that, according to Maxwell, when we write down the result of a measurement, the numerical value multiplies the unit. Hence the name of the unit can be replaced by a kind of algebraic symbol, which is shorter and easier to use in formulae. This symbol is not merely an abbreviation but a symbol which, like chemical symbols, must be used in a precise and prescribed manner.

SI, and hence the use of "km/h" (or "km h-1" or "km·h-1") has now been adopted around the world in many areas related to health and safety and in legal metrology. It is also the preferred system of measure in academia and in education.