The Celestial Mechanics system is simple. It doesn’t change anything about your system of choice. It adds one quick step to non-combat actions, the bid die. It requires one small bit of bookkeeping that parallels tracking hit points, the complications. What it offers in return is greater narrative control for the players and a little bit less responsibility for the gamemaster. The following overview reflects the additions that are made.
Use your favorite d20-based game system as written. Regardless of the game, the edition, or the setting, nothing changes. Instead, Celestial Mechanics adds some to them. Build characters, roll dice, and use abilities exactly as you would under the rules-as-written for that game. Combat, magic, gadgets, superpowers, all work exactly the same. If you have house rules that you’re using, keep those as well.
Make a Die Bid
A bid die reflects the degree of benefits and complications you’re willing to risk in any sort of contested situation. You’re already doing this, sort of, when you enter combat in most d20-based systems. Your character chooses to use a dagger (d4, or whatever your system of choice designates for damage) versus as longsword (a d8). On a successful hit, you roll that die type to see how much damage you’ve inflicted. Many spells do the same thing, whether they do damage, or heal, or have other effects.
For every other situation where you don’t have a polyhedral die assigned to an action, the player gets to bid. If you’re making an ability check, using a skill, or casting a spell that doesn’t normally require a roll, bid a die. Choose a d4, d6, d8, d10, or d12. The larger the die type, the bigger the potential rewards and complications. If you succeed, something good happens. If you fail, something bad happens. The section on die bids further down will cover this in more detail.
Roll a d20 and Add Modifiers
For any situation already covered in the rules you’re using, follow those rules and resolve actions normally. For everything else, use the following guidelines. You’re going to roll to determine the outcome of things that are generally left ambiguous, or up to gamemaster caveat, in most d20-based rules.
The player declares they’d like the character to do something not covered by the rules. Celestial Mechanics now applies. They roll a 20-sided die and add the most appropriate bonus, usually an attribute bonus or a skill modifier. The gamemaster can also apply situational modifiers, in line with what the d20-based rules use. There are no target numbers to compare the total to; all you need to keep track of is whether the total of the roll and modifiers is high or low, even or odd.
High or Low: Success or Failure
Again, this is only for situations not already covered by the d20-based rules. This is an addition to fill in gaps, not a replacement for combat or other existing systems. If the total of the roll plus modifiers is 11 or higher, the character succeeds in what they were attempting to do. If the roll plus modifiers is 10 or less, they fail. No target numbers, nothing else to keep track of. High or low, that’s it.
Even or Odd: Narrating the Outcome
If your total roll plus modifiers is an even number, you get to describe what happens. If your total is an odd number, the gamemaster describes what happens. You can make your character’s successes sound far more impressive, and their failures less painful. The gamemaster can make your victories look like pure luck, and your failures look embarrassing.
This applies to the gamemaster as well. When rolling for an antagonist, if they roll even they get to narrate. If they roll odd, the player whose character the antagonist is attacking gets to narrate. Downplay the attacker’s successes, and make their failures look foolish.