The abacus grew out of early counting boards, with hollows in a board holding pebbles or beads used to calculate. It has been documented in Mesopotamia back to around 3500 B.C.E. The current form, with beads sliding on rods, dates back at least to fifteenth-century China. Before the use of decimal number systems, which allowed the familiar paper-and-pencil methods of calculation, the abacus was essential for almost all multiplication and division. Unlike the modern calculator, the abacus does not perform any mathematical computations. The person using the abacus performs calculations in his/her head relying on the abacus as a physical aid to keep track of the sums. It has become a valuable tool for teaching arithmetic to blind students.