Backgammon Glossary Dictionary: J

Jackpot: A elimination event, usually with a large entry fee, in which only the winner and runner-up receive prize money.
Jacoby Paradox: [Named for Oswald Jacoby, who mentioned it in The Backgammon Book, page 116.] The fact that an improvement in the opponent’s position can make redoubling correct in a position in which the player on roll owns the cube and has one remaining chance to redouble.

Jacoby Rule

[Named for Oswald Jacoby, who proposed the rule.] A rule popular in money play which says that gammons and backgammons count only as a single game if neither player has offered a double during the game. The rule speeds up play by eliminating situations where a player avoids doubling so he can play on for a gammon. See post by Daniel Murphy.
Jacquet: A game once popular in France in which players start at diagonally opposite corners and move around the board in the same direction. See: How to Play Jacquet.
Janowski’s Formula: [Devised by Rick Janowski.] A formula for estimating match equity at a given score. If d is the difference in match score and t is the number of points the trailing player has to go, then the probability of the leading player winning the match is .5 + .85d / (t+6). See also: Neil’s Numbers and Turner’s Formula.
Jellyfish: The first commercial neural-net backgammon program (1994) after TD-Gammon. Website: Jellyfish Backgammon.
Jeopardy: Potential for awkward rolls on a future turn. See also: Double Jeopardy.
Joint Standard Deviation: The standard deviation of the difference between two rollouts: JSD = sqrt(SD1*SD1 + SD2*SD2). A measure of how statistically significant the result is.
Joker: An exceptionally good roll, especially a roll that reverses the likely outcome of the game; a roll much luckier than average.
JSD: Joint standard deviation.
Junior: Affectionate name for a player’s farthest-back checker.

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