Soroban Abacus Construction
The Soroban is composed of an odd number of columns or rods, each having five beads: one bead valuing at five (called a heavenly bead) and four beads valuing at one (called an earth bead). Each set of beads of each rod is divided by a bar known as a reckoning bar. The number and size of beads in each rod make a standard-sized 13-rod Soroban much less bulky than a standard-sized suanpan of similar expressive power.
The number of rods in a Soroban is always odd and never less than nine. Basic models usually have thirteen rods, but the number of rods on practical or standard models often increases to 21, 23, 27 or even 31, thus allowing calculation of more digits or representations of several different numbers at the same time. Each rod represents a digit, and a larger number of rods allow the representation of more digits, either in singular form or during operations.
The beads and rods are made of a variety of different materials. Most Soroban made in Japan are made of wood and have wood, metal, rattan, or bamboo rods for the beads to slide on. The beads themselves are usually biconal (shaped like a double-cone). They are normally made of wood, although the beads of some Soroban, especially those made outside Japan, can be marble, stone, or even plastic. The cost of a Soroban can increase depending on the materials.
One unique feature that sets the Soroban apart from its Chinese cousin is a dot marking every third rod in a Soroban. These are unit rods and any one of them is designated to denote the last digit of the whole number part of the calculation answer. Any number that is represented on rods to the right of this designated rod is part of the decimal part of the answer, unless the number is part of a division or multiplication calculation. Unit rods to the left of the designated one also aid in place value by denoting the groups in the number (such as thousands, millions, etc.). Suanpan usually do not have this feature.
Source : Wikipedia
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