The round starts with four discs in the centre. Take turns placing a disc of your colour.
Every move must trap one or more of your opponent's discs in a straight line (across, down or diagonally) between the new disc and one of yours. All trapped discs flip to your colour. Small dots show where you can play.
If you can't move, you pass and your opponent plays again. When neither player can move, the most discs wins the round. Equal counts make a draw.
Scores carry across rounds, and the starting player alternates each round.
Red discs carry a cross and Blue discs a ring. Keyboard: arrow keys move between squares, Enter or Space plays, Ctrl+Z (Cmd+Z on a Mac) undoes.