Design Diary: 7-Odd-Even
Here’s a neat little dice idea I’m thinking about. If you roll 2d6, the odds of rolling exactly 7 are around 16%. The odds of rolling an odd number other than 7 are around 30%. The odds of rolling any even number are around 50%. Throw in a free reroll and the dice can have a 75% chance of rolling odds.
Those are basically the only odds a player needs to face in an RPG. In a d100 system, a player might have around 10-20% chance of succeeding at something untrained, around a 30% chance of succeeding at something with basic training, around a 50% chance of succeeding at skill that’s core to their character, and around a 75% chance of success if they’re exceptional at a task or very well equipped for it. It wouldn’t be hard to make an RPG engine that uses 2d6 7-odd-even as a core mechanic.
I could see another variant that uses doubles; they also have around a 16% chance of success, so they’re redundant with the 7.
Why would I want to use a mechanic like this? A pair of d6s are the most familiar random number generator in the world. 2d6 and adding modifiers has a board-gamey feel, but 2d6 and calling 7-odd-even feels like traditional gambling. It feels approachable, ordinary, the kind of thing you might do in a break room or at a bus stop. It’s kind of close to the game of craps, although craps is much more complicated. I’ve had some crime and cyberpunk adjacent games on my mind recently, and I think it could work for them. It might even be a more evocative core mechanic for a modern horror game, although I kind of feel like I’ve written enough of those.
There’s a few ways I can see a game using 7-odd-evens. The die rolls could be degrees of success: evens are the worst, sevens the best. Alternatively, a character’s skill could determine what their victory conditions are. Unskilled characters roll for 7, moderate for odds, highly skilled for evens. What I’d really like to do is evoke an even stronger sense of gambling by having a player pick an outcome to roll for, with the less likely outcomes offering better rewards. I’m working on a game that does that, and hopefully I’ll have more to report at some point.
They say you can’t copyright a game mechanic, and I certainly wouldn’t want anyone to, so feel free to take this idea if you like and use it in your own game.