Game Artifacts: The Twenty-Sided d10

Twenty sides, ten numbers.

While inventorying my game collection, I opened up a recently acquired copy of Fantasy Games Unlimited’s solitaire-friendly Star Explorer (1982) to verify the contents.

Map? Check. Countersheet? Check. Log sheets and rulebook? Check. Two six-sided dice? Check. One twenty-sided d10? Um, check?

I’ve worked with all manner of odd polyhedron in my time, from oblong d3s (which have more than three faces) to those hundred-sided “Zocchihedrons” that never quite stop rolling. But a twenty-sider that serves as a d10? New one by me. Silly me—I’ve always used a ten-sided die for a one to ten random distribution.

The typical twenty-sided die, in the shape of an icosahedron, caries the numbers one through twenty, one number per face. The twenty-sider in Star Explorer carries the numbers zero through nine, each number appearing on two faces.

The oddity of this die required special rules for its use in Star Explorer:

1.2 Game Equipment and Scale


6) Dice. Two six-sided dice and one twenty-sided die are included. The twenty-sided die is labelled 0-9 twice. When a roll of 1D10 is required by the rules, players should roll the twenty-sided die, treating a roll of 0 as a roll of 10. When a roll of 1D20 is required by the rules, players should roll the twenty-sided die and a six-sided die. If the six-sided die roll is 1, 2, or 3, the twenty-sided die is read from 1 to (1)0. If the six-sided roll is 4, 5, 6, the twenty-sided roll is read from 11 to (2)0, creating a range from one to twenty. (Players also have the option of coloring in one set of numbers on the twenty-sided die with a fine point felt-tipped marker and reading the colored numbers as 11-20, while the uncolored numbers are read 1-10.)

Ostensibly, providing a single twenty-sider (albeit with oddly numbered faces) to serve as both d10 and d20 proved less expensive than providing a separate d10 and d20. Indeed, SPI stopped providing any dice at all with their games at one point in the 1970’s, blaming the “world-wide petro-chemical shortage,” but likely owing to cost [1]. Whatever FGU’s reason, this little die provides for a great bit of rules verbiage, even if I will break out my own d20 and d10 when I play Star Explorer.

[1] Balkoski, Joseph. "The Perils of Youth: The Lighter Side of SPI." Strategy & Tactics: 128 (Origins 1989), 48-49.

10 thoughts on “Game Artifacts: The Twenty-Sided d10”

  1. 10-sided D10s aren’t completely fair dice, as they are not perfectly symmetrical. There are only 5 shapes that are perfectly symmetrical:

    Tetrahedron – 4 sides
    Hexahedron (cube) – 6 sides
    Octahedron – 8 sides
    Dodecahedron – 12 sides
    Icosahedron – 20 sides

    With these, you can fairly randomize any number from 1 to 6, 8, 10, 12, 20.

    For 7, 9, 11, 13-19, you need to use asymmetric dice, which will always be less fair, and they will need to be correctly balanced in order to be as fair as possible.

    In short, your D10s aren’t as fair as you think they are, and using a D20 to randomize 10, or 5 or even 4 is not done just for financial reasons: the D20 rolls better and is inherently more fair.

  2. Actually, while Chessex may not make them normally I did get one at random in a “Chessex Pound-O-Dice” bag. I thought it was a defective d20 until I looked closely and decided to google “20-sided d10”. So while it may have been a manufacturing error (the bag is factory seconds), Chessex does at least have the ability to make them by accident (though this one looked like it was done intentionally in this manor).

  3. I’m trying to buy some of those for geometrical reasons. There is no such thing as a regular 10-sided polygon. If tou want an honest d10 you need to use a 20-sided polygon numbered from 1-10 twice. The d10 we use normally is just not right.

  4. The use of a plus sign on these special d20s is interesting, and a very easy way to make the die function as either d10 or d20.

    I have to imagine that ten-sided d10s did exist at the same time these these twenty-sided d10s were out there, so I guess cost was the bottom line. Dice are so cheap now, relatively speaking, that it’s not uncommon to buy them by the pound (er, weight, not currency!)

    I’m afraid I don’t have any idea as to where to buy twenty-sided d10s currently. None of the major gaming dice manufacturers like Chessex or Koplow seem to make them.

  5. I love those 20-sided d10s. Hard to come by these days, but great for that “old school” feeling. I think a pair came with the old MERP game. At least, my old MERP GM had a pair… one was opaque navy blue and the other was glow in the dark. Anybody know where to get them today?

    Also, some of those dice had a + sign on half the faces, signifying an “add 10” if you were using it as a standard d20, rather than using the d6.

  6. Thanks for the feedback, David. Interesting to know that this method of using one of these odd d20s predates the FGU guys. It would appear this type of die is more common than I thought.

    Do you happen to recall if there were rules for using the die in those D&D rules? I didn’t pick up my first D&D set until 1983, by which time the boxed sets shipped with normal d20s.

  7. Hi,

    When I started playing D&D in England in 1979-80, these ’20 sided d10s’ were always used, and in the way you described – with a D6.

    When I first saw a ‘real’ d20. I though Christmas had come early


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