France chose the decimal comma because printers had already claimed the dot
Write two and a half as 2.5 in New York and as 2,5 in Paris. The split partly traces to French printing, where full stops were already scattered through Roman numerals to make them readable, so the comma got the decimal job. Britain nearly followed suit in the 1960s before a ministry picked the point.
Separating whole numbers from fractions is older than either mark. Medieval scribes drew a bar over the units digit. Positional decimal fractions first appear in a 10th-century book by the Arab mathematician al-Uqlidisi, who put a short vertical stroke above a digit, building on the Indian numerals that Al-Khwarizmi's translated works carried to Europe. Arab mathematicians later used a small upright stroke between units and tenths, and English writers an L-shaped bar; typesetters found it easier to swap in the comma or full stop they already had.
The earliest known decimal point sits in astronomical tables compiled in the 1440s by the Italian merchant Giovanni Bianchini. John Napier's logarithm tables of 1614 and 1619 used the full stop, and Henry Briggs spread the habit. Italy and many other countries opted for the comma, while English speakers used commas to group digits in threes, creating today's confusing mirror images.
Britain long preferred a raised dot midway up the line, but the international metric system rejected it because mathematicians used that dot for multiplication. During metrication and currency decimalisation, the British Standards Institution argued for the comma and the Decimal Currency Board for the point; the Ministry of Technology settled on the point in 1968. South Africa went metric with the comma, though some English-language papers kept the full stop.
Standards bodies now accept both. The General Conference on Weights and Measures ruled in 2003 that the marker may be a point or a comma on the line, and recommended grouping long numbers with thin spaces instead of either symbol, a practice the US National Institute of Standards and Technology also endorses. Air traffic controllers sidestep the problem by saying the word decimal aloud, and Esperanto, Ido and Interlingua all use the comma.
Source: Decimal separator
