Hex (board element)
The game of hex is played on a board made out of hexagons, or hexes. Here is a 5x5 hex board with 25 hexagons.
At the beginning of the game, all the hexes are empty; players then take it in turns to put their pieces, or stones, onto vacant hexes. A hex can either be empty or it can contain exactly one stone -- once the hex is occupied by one stone neither player can add another stone to that hex, or remove the stone already occupying the hex.
After three moves by each player on a 5x5 board, the game might look like this:
Now 19 hexes are vacant and the remaining 6 are occupied by stones.