. . . "s addition? neither is a function really. they are different ways of configuring digits to represent values. octal uses 8 as its base, meaning it has 8 single digit values, 0-7. decimal uses 10 as its base, meaning it has 10 single digit values, 0-9. with octal, the 8th positive integer (9th non negative integer) is also the first double digit value, 10. the same 0-7 principle applies, and after 1" .