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A vigesimal numeral system has a base of twenty.
Twenty (vingt) is used as a base number in French. E.g.
quatre-vingt means 4 times 20, i.e. 80.
Twenty (tyve) is used as a base number in the Danish
language: Tres (short for tresindstyve) means 3 times 20, i.e. 60; firs (short for
firsindstyve) means 4 times 20 i.e. 80. halvtreds means (3 - 1/2) times 20, i.e. 50; halvfjerds means
(4 - 1/2) times 20, i.e. 70; and halvfems means (5 - 1/2) times 20, i.e. 90.
Twenty (ugain) is used as a base number in the Welsh
language, although in the latter part of the twentieth century a decimal counting system came to be preferred, with the
vigesimal system becoming 'traditional'. Deugain means 2 times 20 i.e. 40, trigain means 3 times 20 i.e. 60.
Prior to the currency decimalisation in 1971, papur chwigain (6 times 20 paper) was the nickname for the 10 shilling
(=120 pence) note.
Twenty may have been the base of the Maya number system, see
Maya numerals and Maya calendar.
In the old British currency system, there were twenty shillings in a pound.
In English, counting by the score has been used historically, as in the famous opening of the Gettysburg Address "Four score and seven years ago...", meaning eighty-seven years ago. This
usage has fallen into disuse in modern times, however.
According to German linguist Theo Vennemann, the vigesimal system is of Basque
(Vasconic) origin and from the Vasconic languages spread to
other European tongues, i.e. many Celtic languages, French and
Danish.
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