for simplicity, consider only numbers with three digits or fewer. to add a pair of numbers (written in arabic numerals), write the second number under the first one, so that digits line up in columns: the rightmost column will contain the ones-digit of the second number under the ones-digit of the first number. this rightmost column is the ones-column. the column immediately to its left is the tens-column. the tens-column will have the tens-digit of the second number (if it has one) under the tens-digit of the first number (if it has one). the column immediately to the left of the tens-column is the hundreds-column. the hundreds-column will line up the hundreds-digit of the second number (if there is one) under the hundreds-digit of the first number (if there is one).