To start each player draws tiles. The number of tiles drawn are: 2 players draw 7 tiles each, 3 players draw 5 tiles each and 4 players draw 5 tiles each. The remaining tiles make up the "boneyard", and are reserved to be drawn upon at need.
The first player places a domino. The first tile played can be of any value. The first double played becomes the "spinner" and can be played off on all four edges. Each player then continues to add a domino to an open end. There may be 2, 3 or 4 open ends.
If a player is unable play, he must draw dominoes from the boneyard until he can make a move. If there are no tiles left to draw, the player must pass.
The object of the game is to make the open ends of the layout add up to 5 or a multiple of five (5, 10, 15, ...). The player who makes such a score receives that number of points. If a double is on an open end, all of its dots are counted.
A round (or "hand") ends when a player played all his tiles, or when the game is blocked (no player is able to make a move). The player with the empty or lightest hand wins the round. In case of a tie, the player with the lightest individual tile wins. The winner of the round scores the number of points in all of his opponents hands.
Dominos io is usually played to 150 points. The first player to reach this score, wins.