Nine white and nine black coins and the red queen start in the centre. White (Light) strikes first, from the bottom baseline; Black from the top. A strike: place the striker anywhere between the two base circles, pull it back and let go. Strike forward, toward the far side, unless a coin sits behind your baseline.
Pocket one of your own coins and you strike again. A coin of the other colour counts for your opponent and the turn passes. Pocket the striker and it is a foul: one of your pocketed coins goes back to the centre (or, if you have none, you owe one and the next coin you pocket goes back), any of your coins pocketed with it go back too, and the turn passes.
The queen may be pocketed only after you have pocketed at least one coin; earlier she goes back to the centre. She must be covered by pocketing one of your coins in the same strike or the next strike of the same turn, or she goes back. The last coin of a colour goes back to the centre while the queen is still on the board.
The board ends when one player has pocketed all nine coins: they score one point for each opponent coin left on the board, plus three for the queen if they covered her (no queen points from 24 up). First to 25 wins, or the higher score after eight boards; the board winner strikes first on the next one. Dues clear with each new board.
Physics: coins bounce off the cushions and each other and slide to a stop, the same way every time, so undo and resume are exact. Undo takes back your last strike (against the computer, its strikes too). Games save automatically.
No online mode for Carrom yet: play across the table or against the computer, which tries out many strikes silently and plays the best it finds in under half a second (fewer tries on Easy, with a shaky hand).
Keyboard: Tab to the board, Left and Right nudge the striker along the baseline, Shift+Left and Shift+Right turn the aim (Shift+Up and Shift+Down fine-tune), Space cycles the power and Enter strikes.