Carnac is a simple fast abstract strategy game for two players. On your turn, you take a Megalith — a piece formed by two cubes joined together — from the common reserve, and place it on the board in a standing position. The turn passes to your opponent if he is willing to tip over the Megalith or if the Megalith cannot be tipped over.

A player owns a square if it is covered by a Megalith showing his color on the upper face. A Dolmen is a group of at least three adjacent squares belonging to the same player. The game is won by the player with the most Dolmens; in case of a tie, the player with the largest Dolmen wins.

All Megaliths are the same: three faces (one square and two opposed rectangular ones) of one color and the other three faces of the other color. The two main aspects of the game are:

• Your move often is also determined by your opponent's choice.
• Having a lot of Megaliths with your own color showing on the upper face is not always a good thing as you might end up with merely one giant Dolmen. The basic aim of the game is building many Dolmens and keeping them separated, while trying to connect your opponent's Dolmens to each other.

It is possible to play Carnac on different board sizes, leading to the use of different strategies to achieve victory.