One side takes X's and the other side takes O's.
Players alternate placing their marks on open spots.
The object is to get three of your marks in a row horizontally, vertically, or diagonally. You can draw or carve marks and they are never moved. Against less than perfect opponents it's an advantage to go first, as having an extra mark on the board never hurts your position. The center is the key square as 4 possible wins go through it. The corners are next best as 3 wins go through each of them.