TITLE: What does $23_4$ mean?
QUESTION [9 upvotes]: I just saw this on a mathematical clock for $11$, i.e $23_4=11$:
$\qquad \qquad \qquad \qquad \qquad$
I guess it is some notation from algebra. But since algebra was never my favorite field of maths, I don't know this notation.
Any explanations are welcome ;-))!
Thanks

REPLY [12 votes]: $4$ is the base, $(23)_4$  means $2 \cdot 4^1 + 3 \cdot 4^0 = 11$. Similarly, if $10$ is the base, then $(23)_{10}$  means $2 \cdot 10^1 + 3 \cdot 10^0 = 23$.