J

Jacoby Rule
Janowski's Formula
Jellyfish
Jeopardy
Joker
Juice
Junior Whopper
Juxtaposed Bars

Jacoby Rule: Players can agree before the game begins that gammons and backgammons will only count as 1 point if the cube has not been doubled by a player during the course of the game. Named after Oswald Jacoby. Practice of the Jacoby Rule forces players to double earlier and speeds up play as very weak positions can be abandoned after a double and the money session can continue.

Janowski's Formula: A formula developed by Rick Janowski that gives an accurate estimate of Match winning chances assuming the players are of equal ability. If D is the difference in scores between the leader and the trailer, and T is the number of points the trailer has to go, the equity for the leader in percentage is: 50 + ((D*85)/(T+6)). Janowski's formula can be used to get an estimate of match equity which can help make doubling decisions. 

Jellyfish: A computer backgammon program that uses neural net technology to create a level of play close to top professionals. Available in various versions with the capability of analyzing matches. Jellyfish has a free player version that can be downloaded at it's website.

Jeopardy: The potential for an awkward roll. A stripped position may lead to jeopardy against an ace point game.

Joker: Any roll that causes a large shift in the odds of winning the game also known as a huge Equity Swing. Jokers are part of what makes backgammon such an exciting game as a single roll can completely change the outcome of a game.

Juice: See Vig. Also the amount of money taken by the house in a transaction. If the juice on a $100 bet was 5%, the house would get five dollars regardless of who wins.

Junior Whopper: A small blunder or mistake.

Juxtaposed Bars: This occurs when one player holds their opponents bar point while the other player is trying to bring checkers home from their own midpoint. When there are juxtaposed bars it is a good example of a holding game