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An abacus translates into physical form the process of simple addition or subtraction, in which numbers are added or removed from a column and any surplus or deficit is carried over to the next column. By performing this function with pebbles or beads, the work is done more by the fingers than the brain and the result can be read directly from the abacus without any need for writing.

The most basic abacus is a decimal one, with each bead in the right-hand column representing '1', each in the next column '10', then '100' and so on. When all the beads in a column have been pushed to the top, they are returned to the bottom and one in the next column is pushed up.

An abacus can be adapted to any proportional system. If calculating pounds and ounces, for example, the right-column will return to zero when 16 has been reached but all the other columns will operate on a normal decimal basis.

Probably the best-known form of abacus in use today is the suan pan ('computing tray') of China. This is decimal, but saves on space and beads by dividing each rod into two parts. The bottom section has five beads, each representing 1; the top part has only two beads, representing 5. The beads in use are pushed up or down against the central bar, along which the number is read.

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