Definition:Decimal Expansion/Historical Note
The idea of representing fractional values by extending the decimal notation to the right appears to have been invented by Simon Stevin, who published the influential book De Thiende.
The idea was borrowed from the Babylonian number system, but streamlined to base $10$ from the cumbersome sexagesimal.
However, his notation was cumbersome: he would write, for example, $25 \bigcirc \! \! \! \! \! \! 0 \ \, 3 \bigcirc \! \! \! \! \! \! 1 \ \, 7 \bigcirc \! \! \! \! \! \! 2 \ \, 9 \bigcirc \! \! \! \! \! \! 3$ for what we now give as $25 \cdotp 379$.
John Napier, in the early $17$th century, appears to have been the first into print with the contemporary notation, although Walter William Rouse Ball suggests that credit for this ought to be due to Henry Briggs.
It was not until a century later, however, that the decimal point came into general use.
- 1938: A. Geary, H.V. Lowry and H.A. Hayden: Mathematics for Technical Students, Part One ... (previous) ... (next): Arithmetic: Chapter $\text I$: Decimals
- 2008: Ian Stewart: Taming the Infinite ... (previous) ... (next): Chapter $3$: Notations and Numbers: The Dark Ages?