We usually write numbers in decimal form $($or base $10),$ meaning numbers are composed using $10$ different

"digits" $\{0,1,$dots,$9\}.$ Sometimes though it is useful to write numbers hexadecimal or base $16.$ Now there are

$16$ distinct digits that can be used to form numbers: $\{0,1,$dots,$9,A,B,C,D,E,F\}.$ So for example, a $3$ digit

hexadecimal number might be $2B8.$