<P> The numbers that may be represented in the decimal system are the decimal fractions, that is the fractions of the form a / 10, where a is an integer, and n is a nonnegative integer . </P> <P> The decimal system has been extended to infinite decimals, for representing any real number, by using an infinite sequence of digits after the decimal separator (see Decimal representation). In this context, the usual decimals are sometimes called terminating decimals . A repeating decimal, is an infinite decimal, that, after some place repeats indefinitely the same sequence of digits (for example 5.123144144144144...= 5.123 144). An infinite decimal represents a rational number if and only if it is a repeating decimal or has a finite number of nonzero digits . </P> <P> Many numeral systems of ancient civilisations use ten and its powers for representing numbers, probably because there are ten fingers on two hands and people started counting by using their fingers . Examples are Armenian numerals, Brahmi numerals, Greek numerals, Hebrew numerals, Roman numerals, and Chinese numerals . Very large numbers were difficult to represent in these old numeral systems, and, only the best mathematicians were able to multiply or divide large numbers . These difficulties were completely solved with the introduction of the Hindu--Arabic numeral system for representing integers . This system has been extended to represent some non-integer numbers, called decimal fractions or decimal numbers for forming the decimal numeral system . </P> <P> For writing numbers, the decimal system uses ten decimal digits, a decimal mark, and, for negative numbers, a minus sign "−". The decimal digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9; the decimal separator is the dot "." in many countries (including English speaking ones), but may be a comma "," in other countries (mainly in Europe). </P>

Why do we use the decimal system (base 10)
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