Decimal separator

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The decimal separator is a symbol used to mark the boundary between the integer and the fractional parts of a decimal numeral.

In the Middle Ages, that is, before printing, a bar over the units digit was used. However, its regular usage and classification is attributed to Muḥammad ibn Mūsā al-Ḵwārizmī, a Persian scientist. Later, a separator (a short, roughly vertical, ink-stroke) between the units and tenths position became the norm. When type-set, it was convenient to use the existing marks called a comma or a period, which is variously called a stop or a dot, or else a point for this purpose.

In France the dot was already in use in printing to make Roman numerals more readable, so the comma was chosen. Many other countries also chose the comma to mark the decimal units position. It has been made standard by the ISO for international blueprints.

English-speaking countries, however, took the comma to separate sequences of three digits. In the US, a period (.), which is called a "stop" or "full stop" in some other countries, was the standard. In the nations of the British Empire, although this could be used as in typewritten material, the point (middle dot: ·), which can also be called an interpunct, was preferred for the decimal separator in those technologies which could accommodate it. This had the advantage of reducing confusion with the countries that used the period to separate groups of digits, but as the middle dot was already in common use in world mathematics to indicate multiplication (for example, in the dot product), the SI rejected this use of this symbol for this purpose. However, the use of the period as decimal point was not banned. British aviation magazines thus switched to the US form in the late twentieth century. When South Africa adopted the metric system, it adopted the , as the decimal marker. For further details, see Comma countries below.

The separator in non-decimal numeral systems may be referred to as a radix point.

Examples of use:

In France, the Netherlands, and much of Latin Europe: 1 234 567,89
In Germany, Belgium, Italy, Romania and much of Europe: 1 234 567,89 or 1.234.567,89 (in handwriting you may also come across 1^·234^·567,89)
In Switzerland (mainly German-speaking Switzerland): 1'234'567,89
In the United Kingdom, United States, Japan, and Mexico: 1,234,567.89 or 1,234,567·89; the latter is more commonly found in older, and especially handwritten, documents nowadays; many UK schools now teach the SI style, which has become official in Australia.
SI style: 1 234 567.89 (dot countries) or 1 234 567,89 (comma countries)
In China, the comma is sometimes used to separate blocks of four digits: 123,4567.89, since in Chinese, there is a word for 10000 (the next new word is for 10⁸, not 10⁶ as in most languages).
In India, due to a numeral system using lakhs (100,000) and crores (10,000,000), an asymmetric distribution of comma separators is often used, for example, 30 million (3 crores) would be written as 3,00,00,000, with commas at the thousand, lakh, and crore levels.