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GameMastery Guide / Advanced Topics / Gambling and Games of Chance

Designing a Gambling Game

Source GameMastery Guide pg. 241
Twentybone is a beat-the-bank game played with 20-sided dice, based on the familiar roll for attacks and skill checks.

The house: Twentybone originated in a casino deep in the back alleys of a major city. The casino’s owner favors exciting games with lots of randomness, and his oft-inebriated patrons appreciate that too. Bones are rattled all night long, and cheaters who sneak in loaded dice find that some of their bones get rattled as well.

The equipment: To play, you need some d20s. A whole lot of them. A dealer and a “dice girl” run the game.

The mechanic: On each round, players can buy any number of d20s they want. The standard peasant’s-wage cost for a d20 is a copper piece, though at some tables a player can spend a silver piece, a gold piece, a platinum piece, or even more for a die. When all dice have been purchased, the players all roll their dice. Then the house rolls a die. Every player die that beats the house die gets paid off with two coins for every one coin it was bought for. For the player, a 1 always loses, and a 20 always wins.

The odds: The player has a 47.75 percent chance of winning. 1’s don’t beat anything, 2’s only beat house 1’s, 3’s beat house 1’s and 2’s, and so on. The exception is player 20’s, which have a special advantage of beating house 20’s.

The payout: The player will win 9.55 coins for every 10 coins he bets. The fact that a player is paid two coins on every win obscures the fact that the player gave over a coin to buy the die. Accounting for this, every successful bet’s payout is 1-to-1.

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Designing a Gambling Game