Link Four is a strategy game where two players take turns stacking red and blue marbles in a 7x7 grid in an attempt to be the first player to establish a linear arrangement of four or more marbles in adjacent grid positions (i.e. to "get four in a row").  Two grid positions are considered adjacent if neither their rows nor columns differ by more than one (thus "four in a row" can be achieved either vertically, horizontally, or diagonally).  The game ends when one player achieves a "link" of at least four marbles, or when the grid has been filled.