\begin{definition}[Definition:Ishango Bone]
The '''Ishango bone''' is a mathematical tool made from a leg bone of a baboon, dating from approximately $25 \ 000$ years ago.
It bears a system of markings which may be tally marks, but may in fact be for a more complex purpose.
It was found in $1960$ by {{WP|Jean_de_Heinzelin_de_Braucourt|Jean de Heinzelin de Braucourt}} in the {{WP|Belgian_Congo|Belgian Congo}} (now the {{WP|Democratic_Republic_of_the_Congo|Democratic Republic of the Congo}}). It was discovered in the area of {{WP|Ishango|Ishango}} near the {{WP|Semliki_River|Semliki River}}.
One row of marks contains the prime numbers between $10$ and $20$, that is, $11$, $13$, $17$ and $19$, which total $60$.
Another row contains $9$, $11$, $19$ and $21$, which also total $60$.
The third row contains marks which appear to represent a method of multiplication by means of a technique involving doubling and halving.
While it is suggested that the markings form a calendar, it is also possible that the markings are coincidental, and perhaps even just put there to make the bone easier to hold.
\end{definition}