But be warned: This rule is good only for sums between 8₁₀ and 15₁₀ inclusive, or between 10₈ and 17₈. (For sums of 7 and less you need no rule, since the octal and decimal numbers up to and including 7 are identical. Notice that the sum of any two single digit octal digits cannot be higher than 16₈, just as the sum of any two single decimal digits cannot be higher than 18₁₀.)

So, armed with this rule we can handle the addition of two octal numbers of any number of digits by adding one column at a time just as we do in decimal arithmetic. When a column sum is 10₈ or more, we have 1 to carry to the next column, of course. For example, the addition of 36₈ + 73₈ would go as follows:

You will note that even with a “carried 1” the sum of each column of a two-number addition cannot exceed 17₈, so our rule for adding two octal digits is good for adding three octal digits in this special case where one of the digits is one.