The Hindu - Arabic positional numeral system represents a number very simple by recording in order the number of counters belonging to the various lines of the abacus. The symbol 0 stands for a line with no counters on it. Our present addition and subtraction patterns, along with the concepts of "carrying over" and of "borrowing" may have originated in the processes for carrying out these operations on the abacus. Since, with the Hindu-Arabic numeral system, we are working with symbols instead of the actual counters, it becomes necessary either to commit the simple combinations to memory or to have recourse to an elementary addition table.