I'm trying to adapt GURPS 4th edition's system creation system into a Python script and I'm trying to understand the dice notation.

In particular:

Roll 1d-4 (minimum 0) to determine the number of major moons orbiting a terrestrial planet. If the planet has no major moons, it will have 1d-2 (minimum 0) moonlets.

Modifiers (for both rolls): Do not roll if the planet is within 0.5 AU of the primary star, -3 if the planet is between 0.5 AU and 0.75 AU of the primary star, -1 if the planet is between 0.75 AU and 1.5 AU of the primary star, -2 if the planet is Tiny, -1 if it is Small, +1 if it is Large.

I've not seen the "1d-4" notation before, so my assumption would be that you role 4 sided dice (or in my case, get a random value between 1 and 4 inclusive) and apply modifiers to the result, e.g.: For a Small planet at 0.8AU from it's primary I would: Get value between 1-4 Apply -1 to that value (between 0.75AU and 1.5AU) Apply -1 to that value as the planet is Small

The catch here, is that, if I'm reading the notation correctly (which I don't think I am), 1d-2 means pick a value of 1 or 2 then apply modifiers, which will always results in 0 or less.

How do you read 1d-4 and 1d-2 ?

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    \$\begingroup\$ Welcome to RPG.SE! Take the tour if you haven't already and see the help center or ask us here in the comments (use @ to ping someone) if you need more guidance. Good Luck and Happy Gaming! \$\endgroup\$
    – Someone_Evil
    Jan 27, 2021 at 4:13
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    \$\begingroup\$ I'd be interested in seeing that Python script, once you're finished with it, if you're willing to share it. \$\endgroup\$
    – nick012000
    Jan 28, 2021 at 3:26
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    \$\begingroup\$ Coming from D&D, my immediate reaction to the title was that "roll 1d-4" means "roll one die with negative four sides." Maybe it's an antimatter die, or a die with the normals facing inward. \$\endgroup\$ Jan 28, 2021 at 20:43
  • \$\begingroup\$ FYI There's an implementation complete with a CLI and web front end here: github.com/tschoppi/starsystem-gen Perhaps it can help inspire you to make yours better - although I think we did a decent job, there's always room for improvement! \$\endgroup\$
    – MarkZ
    Mar 9, 2021 at 15:45

2 Answers 2


GURPS uses 6-side dice for practically everything. It's more economical to refer to them as simply "d" rather than "d6."

GURPS uses six-sided dice only […] GURPS uses the "dice+adds" system […] "3d-3" means "roll 3 dice and subtract 3 from the total." (GURPS 4e Basic Set: Characters, p. 9)

So you should read "1d-4" as meaning "1d6-4" (roll one six-sided die, then subtract 4 from the result) in standard notation, et cetera.

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    \$\begingroup\$ And to be explicit: d6-4 means there are 1-2 moons 33% of the time. Or 1-4=no moons, 5=1 moon, 6=2 moons. A small planet (-1 to the roll = 1d-5) has a moon on a '6'. A close planet (-3 on the roll) never has a moon and has one moonlet on a 6 (1d-2-3) \$\endgroup\$ Jan 27, 2021 at 16:45
  • \$\begingroup\$ The mention of 'minimum 0' seems to make this clearly the correct interpretation. If there were no chance of negative numbers there would be no reason to specify 'minimum 0'. I've never seen any dice with negative or zero as values. Even without knowing anything about GURPS I would have assumed a d6, but it could have been possible they used d8 or d10 by default I guess. \$\endgroup\$ Jan 27, 2021 at 21:54
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    \$\begingroup\$ @JasonGoemaat For the record, the dF is a die with a value range of {-1,0,+1}, but GURPS never used those. GURPS uses a d6 (or some number thereof) and uses modifiers like addition/subtraction/multiplication to arrive at the desired ranges of results. \$\endgroup\$ Jan 27, 2021 at 23:10
  • \$\begingroup\$ GURPS only uses standard d6's. I've played GURPS since first edition, and I don't remember ever seeing any other kind of dice being rolled. In responce to @vicky_molokh-unsilenceMonica the dF is a Fate (or Fudge) die; great for that game. \$\endgroup\$
    – NomadMaker
    Jan 28, 2021 at 21:44

Yes, "1d-4" or just "1d4" is a single die with 4 sides- a regular tetrahedron.

"1d-2" or "12" is a single "die" with two sides. That's a bit peculiar- you're actually flipping a coin!

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    \$\begingroup\$ This is incorrect, please see the other answers. \$\endgroup\$ Jan 27, 2021 at 18:43

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