The Jacoby rule, invented by Oswald Jacoby, says:
In money play, the game winner receives no extra credit for gammons or backgammons unless the cube has been doubled at least once.
The Jacoby rule is standard for money play almost everywhere, although it’s still a good idea to establish that you’re using the rule when you start a money play session.
The sole purpose of the rule is to speed up the game – add a little more excitement — finish this game and move on to the next. The Jacoby rule is never used in match play. The rule avoids those long games where, having missed the market by a mile, one might play on with little chance for either a gammon or a loss, or where, in match play with a centered cube, a position is too good to double.
The Jacoby rule has some influence on both checker play and cube decisions, mostly on checker play.
It encourages more aggressive checker play to achieve a gammonish doubling position. The idea (simply put) is that if the strategy works, you double and win four points; if it fails, you pass and lose one, since unless the cube is turned you can’t lose a gammon.
The rule’s affect on cube decisions is harder to pin down. It seems sensible that there will be positions which are not good enough to double in a match but may be money doubles, if much of the doubler’s equity comes from gammons won, even if the doubler’s overall winning chances aren’t terrific. In actual money play the rule probably encourages typical players to double — not just earlier – but too early, for fear of missing out on four points by not “activating gammons.” But against opponents who are known to be afraid to take a double in a gammonish position, or to redouble if the game turns around, the “early” double can still be a money winner.