Always check the rules of the backgammon game your wish to play online, because they do vary. In some cases, the two players will even be allowed to decide among themselves (before the game) whether they want to put certain rules into play or not.
If you agree to allow beavers, it affects the rules for the doubling cube. If one player X is using the doubling cube, and player Y accepts the double, player Y may instantly redouble while keeping possession of the cube. An instant redoubling without giving up the cube is called a beaver.
If you agree to allow racoons, it affects the rules for the doubling cube. By allowing racoons, you make it permissible for a player to redouble the beaver right away. This is called racooning.
In a game of backgammon where the Crawford rule is in play, the doubling cube can not be used when a player is one point away from winning the entire match. The Crawford rule makes it impossible to raise the stake for one single game in the match.
Of course, having the Crawford rule in play for single games would make no sense. It is only of interest for match play.
If the Crawford rule is in play, the Holland rule can also be put in play. The Holland rule stipulates that after the Crawford game, none of the players can double until at least two rolls have been played by each side.
This rule is named after Tim Holland. It was popular in tournament play in the 1980’s but is rarely used today.
If the Jacoby rule is in play, it motives the players to use the double cube. If the doubling cube has not been used at least once during a game, and the Jacoby rule is in play, gammons and backgammons do not count.
Putting the Jacoby rule into play tend to speed up the end of a game where one player is at a distinct advantage.