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The commonly used number system, also known as denary, in which each place has a value 10 times the value of the place at its right. For example, 4327 in the decimal (base 10) system is shorthand for (4 × 103) + (3 × 102) + (2 × 101) + (7 × 100), where 100 = 1. Similarly, fractions may be expressed by setting their denominators equal to powers of 10.

The word decimal comes from the Latin decimus for "tenth." The verb decimare, literally "to take a tenth of," was used to describe a form of punishment applied to mutinous units in the Roman army. The men were lined up and every tenth soldier was killed as a lesson to the rest. From this custom comes our word "decimate," which we use more loosely – in fact, incorrectly – to indicate near-total destruction. The Latin decimare was also used in a less ferocious sense to mean "to tax to the amount of one tenth." However, the usual word describing a one-tenth tax in English is tithe, which comes from the Old English teogotha, a form of tenth.

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