Nackgammon is a variant of backgammon developed by Nack Ballard.
The rules of nackgammon are identical to backgammon, except that the starting board position is different. In nackgammon, you have four checkers all the way back--two on the 24 point (like backgammon), and two on the 23 point. The two extra checkers are taken from your six point and thirteen point (which then have only four checkers each).
To play a nackgammon match, choose "Nackgammon" from the popup menu in the Match section of the Invite window.
GridGammon holds frequent nackgammon tournaments, or you can play nackgammon against the GG robots (only GridGammon members may play the robots).