“Decimal comes from decimus, Latin for 'tenth.' Humans count in tens because we have ten fingers. If we had eight fingers, mathematics would look different, but it would work exactly the same.”
Decimalis in medieval Latin means 'of or pertaining to tenths,' from decimus (tenth), from decem (ten), from PIE *dekm (ten). The base-ten number system is almost certainly biological in origin — humans have ten fingers, and finger-counting is the oldest computational technology. Not all cultures used base-ten: the Babylonians used base-sixty (which is why we have 60 minutes in an hour), and the Maya used base-twenty (counting fingers and toes).
The Hindu-Arabic decimal system — the system we use, with digits 0-9 and positional notation — was developed in India by the seventh century CE. Brahmagupta's Brahmasphutasiddhanta (628 CE) used zero as a number, not just a placeholder. Al-Khwarizmi brought the system to the Islamic world around 825 CE. Fibonacci brought it to Europe in 1202. The transition from Roman numerals to Hindu-Arabic decimals took centuries and met fierce resistance from those who thought Roman numerals were safer from fraud.
Decimal fractions — numbers like 3.14 — were not obvious. Integer decimals (counting in base ten) are ancient. Decimal fractions required the invention of the decimal point. Simon Stevin advocated for decimal fractions in 1585. The decimal point itself appeared in different forms — John Napier used a period in 1617, and the convention spread. Some countries use a comma instead of a period. The notation is arbitrary. The mathematics is not.
The metric system (1795) made decimals the official language of measurement. Meters, liters, grams — all divide and multiply by powers of ten. Countries that use imperial units (miles, gallons, pounds) still use decimal arithmetic. The system is so natural to humans that it is hard to imagine alternatives, but the Babylonian base-sixty persists in time and angle measurement, and base-two (binary) runs every computer. Decimal is dominant because of anatomy, not mathematics.
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Today
The decimal system is so embedded in human civilization that it is invisible. We count in tens because we have ten fingers. It is anatomy masquerading as mathematics. Other bases work just as well — computers prove that daily with binary. But the ten-fingered primate built a world in base ten, and the world cooperated.
The Latin said tenth. The fingers said ten. The mathematics said it does not matter. Any base works. But we use the one our bodies gave us.
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