Why no d18's?

[JohnSnow]

They are platonic solids. That's one reason there were no d10s in the set Gygax bought. While a d10 does have faces the same size and shape, it does not have same number of faces meeting at each corner.

The faces on the d10 are also not equilateral polygons, unlike all the others. I won't get too math-y, but there's a reason the faces on the platonic solids are either an equilateral triangle (d4, d8, d20), square (d6) or pentagon (d12).

 

[Blue Orange]

I suspect d3 was close enough to 'roll a regular non-RPG die and divide by 2'...also they needed something below d4, whereas with d5 you could either use d4 or d6 .

It's probably also easier to make fair and fair-looking platonic solids, as the others will look asymmetric even if not biased.

I always figured the DCC dice were to give the same feeling of playing with 'weird dice' people had when they first brought out polyhedral dice in the 1970s.

I collect dice, and have a d30, d100, d5, d3, d7, d34, and d16 and d24. (There was a pack of 'why not'? dice at the Compleat Strategist in Manhattan, and I picked them up.) The odd-numbered ones tend to roll funny, as RogueJK says.

As an aside, the highest-symmetry crystal in mineralogy is the hexoctahedron, which is basically a d48.

Crystal system - Wikipedia

en.wikipedia.org

There are potential designs for d16s and d24s in there...

 

[Umbran]

Is it an oversight? Is it because it's considered too close to the d20 to make a difference?

I'd say that latter. After all, on a d20, each side as a 5% chance of coming up. On a d18, it would be a 5.55555% chance. Half a percent really isn't worth the difference.

EDIT:
And on a tangentially related note, how come RPGs use d3's but not d5's; it seems like those should be the same principle as each other; one's a d6 divided by 2 and rounded up, the other's a d10 divided by 2 and rounded up

Same principle. You go from a coin flip (50% chance) to a d3 (33.333% chance) to a d4 (25% chance).

a d5 is a 20% chance. a d6 is an 18% or so - a 2% difference isn't that big a deal most of the time.

 

[aramis erak]

A fair and even d18 is pretty easy - an offset dipyramid, just like the modern d10, but a nonagonal rather than a penatagonal dipyramid.
d16's are usually octagonal dipyramids. (no offset needed)
d14's as dipyramids are also offset, but are offset septagonal dipyramids.

(Note: I prefer septagon to heptagon, both of which mean a 7-sided 2d shape)

 

[Bohandas]

It doesn't look like all of the faces on a d18 have the same chance of being rolled. The two distinct sets of faces, six of one and 12 of the other, each which will have different odds than each other.

Actually it's eighteen and eight. But the eight triangular faces are unmarked and seem to have protrusions to prevent the die from landing on them

 

[Wasteland Knight]

d18 is too close to a d20 to drive its usage.

 

[Fenris-77]

Because d18s are silly? Not to throw a wooden shoe in the thread, but I can't think of a single reason I'd want one. I'd probably not buy a game strictly on the basis that it wanted me to uae a d18. Anyway, carry on.

 

[Tonguez]

Because they are pointless for anything other than curiosity value

it is possible to get d3, d5, d7, d14, d16, d18, d22, d24, d26, d28, d30, d48, d50, d60, d100, d120

 

[CleverNickName]

I wish I could remember where I saw them, but once upon a time I saw a set of special dice out there that could give interesting combinations. They were basically for people who didn't want to do basic division in their heads.

The set had five d12s in it, all of them numbered differently: one of them was numbered from 1 to 2 six times (making it a d2), numbered from 1 to 3 four times (making it a d3), from 1 to 4 three times (making it effectively a d4), from 1 to 6 twice (yep, a d6 replacer), and from 1 to 12 once (your standard d12).

There were five d20s also: one was numbered from 1 to 2 ten times (another d2), 1 to 4 five times (a d4), 1 to 5 four times (a d5), and from 1 to 10 twice (a d10), and of course from 1 to 20 (the basic d20).

 

[Bohandas]

Because d18s are silly? Not to throw a wooden shoe in the thread, but I can't think of a single reason I'd want one. I'd probably not buy a game strictly on the basis that it wanted me to uae a d18. Anyway, carry on.

Admittedly a d16 would be more useful. There's too much of a gap between the d12 and the d20. But d18 was the one that I came across as having apparently been a thing for a while and yet was left unused by RPGs