Definitions for "Jacoby Rule"
A gammon or backgammon game can only be scored if the cube has been doubled. Used principally in money games and not match play. page top page top page top
There cannot be a gammon or backgammon until there is has been a doubling.
The Jacoby Rule means that gammons and backgammons will count for double and triple points only in the case that at least one doubling has occurred during the game.