The
abacus as shown in the figure consists of a rectangular
wooden frame which is divided lengthwise into two
unequal parts by a horizontal beam. It may have nine,
eleven , thirteen, seventeen or more ordinate columns
of moveable beads usually made of wood. The number
of beads in each column is five, one above the beam
and four below. For convenience the beads above the
beam are called upper beads and those below it lower
beads.
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