Mental Morris is a two player abstract played entirely in the mind. It is however played on a mental board of 3x3 squares with three pieces each. The object is the game is to form a straight line of your pieces, either diagonally or orthogonally. If a player makes an illegal move, the opponent wins.
The board is numbered from top to bottom like a num pad. 1,2,3 on top, 4,5,6 on the next row and 7,8,9 on the final row.
Players initially place their three pieces on free squares by calling out the number of the square where they want their piece to be. After that players call out moves by naming the square of the piece they want to move, and then the square they want their piece to move to. Say it as one number, i.e. "fifty eight", (58). Pieces may move only one space in an orthogonal direction. Pieces may not share a square, and jumping over pieces is not allowed.
—description from the designer