10/Historical Note

Historical Note on $10$ (Ten)
made the obvious statement that $10$ is the usual number base because we have $10$ fingers. This view was echoed by the Roman poet.

It is noted that not all cultures use $10$ -- some use $5$ (based on the number of fingers on a single hand) and some use $20$ (based on the total number of fingers and toes).

The number $10$ was considered holy by the Pythagoreans, who set great score to the fact that $10 = 1 + 2 + 3 + 4$.

They knew about $8$ celestial bodies:
 * Sun, Moon, Mercury, Venus, Earth, Mars, Jupiter, Saturn

and also believed in the.

In order to satisfy their belief that $10$ was such a special number, they hypothesized the existence of a which orbited the  in such a position as to be hidden from us by the Sun.

The number $10$ is expressed in Roman numerals as $\mathrm X$.

It has been suggested that this originates from:
 * joining up two $\mathrm V$s, each representing $2$ hands held up showing the full $5$ fingers

or:
 * an abbreviation for $10$ tally marks, with a line struck through them to indicate that $10$ had been reached.

Also see

 * Definition:Tetractys