# Why does a d10 count from 0 to 9?

I have recently purchased (in Poland, if that matters) a set of dice, from d6 to d20. I'm a complete newbie (if not ignorant) to RPG in general, dice, and this sort of technical stuff in practical, mainly because I was playing RPG very little in my life. But, since:

• d6 allows to pick any number between 1-6 ("counts" to 6),
• d8 allows to pick a number in 1-8 range,
• d12 gives a number between 1 and 12,
• d20 goes in 1-20 range, then...

...how it is possible, that d10 allows me to pick any number between... 0-9?

Did I just buy some weird dice or am I missing something obvious? As I said, I haven't been playing RPG or board games in general too much, but I have never seen or even heard of any dice that allows to pick zero.

• I've modified your title to be more accurate with your question. If your question is different than that, feel free to roll back to a previous edit. – Premier Bromanov Jan 29 '16 at 20:27
• Also, in case you were still wondering, it does not matter that you bought them in Poland. – Premier Bromanov Jan 29 '16 at 21:32
• "how it is possible?" It's obviously possible to paint the numbers 0-9 onto a 10-sided object. Likewise, it's perfectly possible to produce a 10-sided die with faces labelled 2, 3, 5, 7, 11, cat, 13, 17, 19 and 23. – David Richerby Jan 30 '16 at 19:52

There are 10-sided dice numbered 1-10. You should be able to find some easily if you search. But it's extremely common, especially among wargamers, to use 0-9.

There's a simple reason for this: Percentages.

Many systems use a percentile chance of something happening. By rolling two dice labelled 0-9, in different colours with one die chosen as the tens digit, you get a number from 0-99 - a percentage chance (often, but not always, counting 00 as 100).

This is much less readable if your dice are labelled 1-10, because you have to treat the 10 as 0 on both dice.

• There are also dice labeled 00-90, which forces them to be used as the tens place rather than using 2 similar d10s – Premier Bromanov Jan 29 '16 at 20:25
• It's probably worth mentioning in games that do use 1-10 instead of 0-9, the 0 is read as 10 - it's game specific, and the rules should call out which way to read it. There's also manufacturing issues involved - if all the digits are single (or all doubles) it is easier to align the tooling with the faces of the die. – T.J.L. Jan 29 '16 at 20:28
• An example of a game (albeit not an rpg) that uses 1-10 dice is the Pathfinder Adventure Card Game. The d10s are always used as such and never as d%, so from the second set onwards, they've been using 1-10 dice to avoid having to explain to players unfamiliar with d10 how to read them – 3Doubloons Jan 30 '16 at 0:58

If a game asks you to roll a ten-sided die, you treat the "0" face as "10." This gives you the value range you'd expect: 1 - 10, matching the other dice.

This is extremely common. Usually if a game wants you to treat that face as a zero result, the game will call it out explicitly.

There are a couple reasons for this.

One is simply cosmetic. With nine single-digit faces and only a single two-digit face, using 0 instead of 10 allows them to print all the faces at a similar large font size.

The other is, as others have noted, to make the die work better for rolling "percentiles." In this case, you roll two ten sided dice. One is labeled (0, 1, 2, ... 9). The other is labeled (00, 10, 20, ... 90).

When read together, the two dice give you results from:

(00) (0)

to

(90) (9)

Or, (depending on the game) from:

(00) (1)

to

(00) (0)

With (00) (0) being treated as 100.

• Ahhhh the user avatars! – SevenSidedDie Jan 29 '16 at 20:48
• @MontyHarder You're thinking of the technical term percentile, used for grouping demographic data (e.g. I am in the first percentile of users on this site! Woo!). The definition here is looser, it basically means "thing you roll to get a percentage." Charts are often arranged from 01 - 00 (at least in D&D). This has the advantage of being able to say "I have a 1% chance of rolling 1 or less; a 10% chance of rolling a 10 or less; a 100% chance of rolling a 00 or less; etc." – AceCalhoon Jan 29 '16 at 21:51
• @MontyHarder This is simply the idiom in common use. File it in with danishes and guinea pigs. – AceCalhoon Jan 29 '16 at 22:04
• @MontyHarder RPGs have all kinds of domain-specific jargon, obtained by invention and repurposing, and the adjective “percentile” (derived from the noun you mention, but only in form, not meaning) is simply one such term. As much as it makes no sense outside the domain of RPGs/wargaming, within it they are correctly called “percentile dice” and there's no, er, percentage in anyone trying to change it, since it's unambiguous and otherwise perfectly cromulent. – SevenSidedDie Jan 29 '16 at 23:45
• It's not just idiomatic; it's correct. A quick Google search shows there isn't one precise definition, even in statistics, and there are multiple definitions that say the word can mean either the 99 (or 101) division points, or the 100 division groups. Also, note that there is both a 100th and 0th percentile, depending on the precise definition you're using. And that doesn't even include the adjective definitions. – MichaelS Jan 30 '16 at 7:55

Ten-sided dice, often sold in pairs (the two are different colors), are most commonly used to roll percentile values -- that is, the two dice are rolled together, and the result read by multiplying one previously designated die as ten times its reading, and adding the other's reading. This gives a result from 00 to 99. Because of this, most 10-sided dice are marked 0 to 9 rather than 1 to 10 (sometimes, one die in a percentile set will be marked from 00 to 90).

Percentile dice are sometimes (more commonly in the past, before 10-sided were common) sold as a pair of 20 sided dice, marked from 0 to 9 twice.

• It's worth pointing out that percentiles run from 0 to 99; there is no "100th percentile". So if something has a 50% chance of being successful, that means you have to roll under 50 on your percentile dice, or 00-49, which are a total of 50 of the 100 possibilities. If something has 0% chance, you can't roll under 0, and if it's 100% chance, anything you rolled would be under 100. It all makes sense viewed this way. – Monty Harder Jan 29 '16 at 21:30
• @MontyHarder what people usually do when rolling percentiles is treating the 0+00 as a 100, so a chance of 50% is 01-50 and the other 50% is 51-100 (instead of 00-49 and 50-99) – Zachiel Jan 29 '16 at 23:02

Most d10s (as that die is usually called in English) count the way the actual decimal system does, starting at 0. You can think of 0 as being 10 if you like; it's basically the same thing.

The only time this gets very complicated is if you need to roll 2 d10s as a d100 (percentile) die. But as long as you maintain a consistent system, you're fine. (Counting from 00 to 99 is probably the best in principle.)