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Abacus is played on a hexagonal board with five cells per side. Smaller boards can be used for a shorter game, while larger boards can be used for a deeper game.

The goal of Abacus is to have fewer stones than the opponent when the board is completely filled with stones.

No draws are possible in Abacus.

The game begins with an empty board.

Each player has an allocated color: Black or White.

A group of stones is defined as a set of same-colored stones touching each other.

Starting with Black, players take turns making one of the following moves:

• Place a stone of their color on any empty cell.
• Move their stone along a straight line, detaching it from its original group to form another group of a smaller size than the original. The moved stone must land on an empty cell and it may not jump over any stones of either color. The newly formed group must contain at least two stones. Removing the stone from the original group may split it into several parts.

The diagrams below show examples of legal moves for Black:

Before: the highlighted stone belongs to a group of four.	After: the stone joins a group of two to form a group of three.

Before: the highlighted stone belongs to a group of three.	After: the stone forms a group of two, leaving a singleton behind.

Before: the highlighted central stone belongs to a group of three.	After: the stone joins the isolated stone to form a group of two, leaving two separate singletons behind.

The diagram below shows three illegal destinations for the marked Black stone on C4:

A2 is illegal: the moved stone would form a singleton. C2 is illegal: the moved stone would join the stones on D2 and D3 to form a group of three, which is not smaller than its original group of three. F4 is illegal: although the moved stone would form a group of two with the stone on G4, the White stone on E4 blocks the path. Stones may not jump.

When the board is full, the player with fewer stones wins.

• Abacus on BoardGameGeek