Standard
Can go out on a double
You may go out by playing a double as your last domino. The round ends immediately, so you do not have to cover it.
Ending a round
A round ends in one of two ways: a player plays their last domino, or the boneyard is empty and every player passes in a row without anyone playing a domino.
When the boneyard is empty, keep taking turns. Play a domino if you can. If you cannot, mark your train if it is not already marked and pass. Any domino played starts the pass count over.
Stricter Option
Can't go out on a double
You cannot go out by playing a double as your last domino. You may play a double, then go out by playing a last domino that covers it.