Abacus is an instrument used in performing arithmetic calculations. It's a mechanical aid used for counting; it is not a calculator in the sense we use the word today. The person operating the abacus performs calculations in their head and uses the abacus to keep track of the sums, the carrys, etc.
The device evolved from a simple need to count numbers. Merchants trading goods not only needed a way to count goods bought and sold, but also to quickly calculate the cost of those goods. Until numbers were invented, these counting devices were used to make everyday calculations.
It consists essentially of a tablet or frame bearing parallel wires or grooves on which counters or beads are moved. A modern abacus consists of a wooden frame with beads on parallel wires, and a crossbar oriented perpendicular to the wires that divides the beads into two groups.
Each column -that is, each wire- represents one place in the decimal system. The column furthest to the right is the units column; the one to the left of this is the tens column; and so on. In each column there are five beads below the crossbar, each of which represents one unit, and two beads above the crossbar, each of which represents five units.
Many early civilizations used the abacus. In ancient Roman culture it was a sand-covered wax tablet, marked table, or grooved table or tablet. A simplified form of abacus was used in medieval England. It consisted of a tablet ruled into spaces representing the positions of the counters; coins, buttons, stones, or other small objects were moved to make the calculations.
The chequered tablecloth, from which the name Exchequer is derived, was originally a calculating device like the ruled tablet. The abacus is still widely used by business people and accountants in China and Japan. Skilled users can obtain results more quickly than is possible with an electronic calculator.
The earliest counting boards are forever lost because of the perishable materials used in their construction. We can, however, make educated guesses about how they were built. The simplest counting board probably involved drawing lines in the sand (the space between 2 lines would represent the units 10s, 100s, etc.) and placing small pebbles within those lines, as place holders representing numbers. With the need for something more durable, wooden boards with grooves carved into them, were then created. The wooden boards then gave way to more permanent marble (or other stone) and metal tablets.
The Salamis Tablet
The oldest surviving counting board is the Salamis tablet, used by the Babylonians circa 300 B.C., discovered in 1899 on the island of Salamis. It is a slab of marble marked with 2 sets of eleven vertical lines (10 columns), a blank space between them, a horizontal line crossing each set of lines and Greek symbols along the top and bottom.
The evolution of the abacus can be divided into three ages: Ancient Times, Middle Ages, and Modern Times. The timeline below traces the developing abacus from its beginnings circa 500 B.C., to the present.
During Greek and Roman times, counting boards, like the Roman hand-abacus, that survive are constructed from stone and metal. (As a point of reference, the Roman empire fell circa 500 A.D.)
The Middle Ages
Wood was the primary material from which counting boards were manufactured; the orientation of the beads switched from vertical to horizontal. As arithmetic (counting using written numbers) gained popularity in the latter part of the Middle Ages the use of the abacus began to diminish in Europe.
The abacus as we know it today, appeared circa 1200 A.D. in China; in Chinese, it is called suan-pan. On each rod, this classic Chinese abacus has 2 beads on the upper deck and 5 on the lower deck; such an abacus is also referred to as a 2/5 abacus. The 2/5 style survived unchanged until about 1850 at which time the 1/5 (one bead on the top deck and five beads on the bottom deck) abacus appeared.
Circa 1600 A.D., use and evolution of the Chinese 1/5 abacus was begun by the Japanese via Korea. In Japanese, the abacus is called soroban. It is thought that early Christians brought the abacus to the East. The Russian abacus, invented in the 17th century and still in use today, is called a schoty.
There have been recent suggestions of a Mesoamerican (Aztec) abacus (Nepohualtzitzin), circa 900-1000 A.D., where the counters were made from kernels of maize threaded through strings mounted on a wooden frame.