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e.g. "d6", for a 6-sided die.

Put the letter after the number, and that makes sense. That's an abbreviation for "six-dimensional".

When you've got a tuple, for example, you just call it an "n-tuple". Why not an "n-die"?

I read Wikipedia but it's not enough information for me to comprehend the convention. I don't understand why an abbreviation is necessary, especially verbally. "Die" is only one syllable, and three letters.

Note: This sort of naming is common in source code, as most programming languages don't allow type or variable names to start with numbers. (See Table 2.3 here for examples. An int3, for instance, is a data type that comprises three integers. It should actually be called 3Ints, but that name won't compile.)

You might name a type, in code, Die6, but in English, a reversal of the position of the number is standard. e.g. As a Vector3 is really a "3-vector", so I'd expect a "Die6" to represent a "6-die".

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    Related question over at rpg.se: "Dee-twenty" and "die-twenty" pronunciations, might actually give a thorough answer here. – Thomas Markov Nov 27 '20 at 20:46
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    I'm confused: what do you (Jessy) mean when you say "Put a letter after the number, and that makes sense." How does "6d" intrinsically make any more sense than "d6"? And while that is a valid abbreviation for "six-dimensional", what does six-dimensionality have to do with this question? – David Z Nov 28 '20 at 6:45
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    Comments are for clarification, not extended discussion, which is generally more appropriate for the Board & Card Games Chat. – murgatroid99 Dec 1 '20 at 19:21
  • By my reading, the second comment I left is a valid clarification question, so I do not believe that deleting it is warranted. Aside from that, I have deleted the new comment after mine because it is hostile and insulting, and that is inappropriate for this site. Please refrain from making such comments in the future. – murgatroid99 Dec 2 '20 at 3:36
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    I’m voting to close this question because primarily, it does is attract useless activity from butthurt people who don't spend enough time reading the question. It will continue to waste people's time in the future. – user34579 Dec 2 '20 at 19:56
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It's a Dungeons & Dragons convention

The "d" dice notation originates with Dungeons & Dragons, which innovated in the use of multiple types of polyhedral dice and often requires multiple such dice to be rolled at once. The frequency with which rolls are required necessitates a dice notation in order to present it in an abbreviated form. It dates back at least as far as 1977.

Dice notation did not appear in the original 1974 D&D box set, requiring writers to type it in full. For example:

Prior to the character selection by players it is necessary for the referee to roll three six-sided dice in order to rate each as to various abilities, and thus aid them in selecting a role.

In a 2013 article, RPG historian Jon Peterson traces it to the fanzine Alarums & Excursions, issue #1 (June 1975), in the article Dice as Random Number Generators by Ted Johnstone, an alias for writer David McDaniel:

It is the intent of this monograph to examine the statistical distribution of different numbers and combinations of the five types of dice available to us: 4-sided, 6-sided, 8-sided, 10-sided and 12-sided, referred to hereinafter for convenience's sake as D4, D6, D8, D10, and D12. [...]

To demonstrate this thesis in its simplest form, compare one D12 against two D6.

This illustrates the original meaning and purpose of dice notation:

  1. D-number (e.g. d8) is an abbreviation for the benefit of brevity in written form. Most gamers in 1977 were not mathematicians or computer programmers, and the Web did not exist for research. As such, they would not have been influenced by these concepts, and terms like "n-tuple" would have been unknown to them. Johnstone, or someone who influenced him, simply decided that "D-number" was a sufficient syntax.
  2. The syntax for multiple dice, e.g. 2d8, literally means two "d8"s. One eight-sided die is d8; 2d8 is literally two of them.

Use in D&D

The earliest instance of its use in D&D which I can find is Dragon Magazine #7 (June 1977), in the article What To Do When the Dog Eats Your Dice, by Omar Kwalish, an alias of the magazine's editor Tim Kask. The AD&D Monster Manual (1977) did not adopt dice notation, but the Player's Handbook (1978) officially introduced dice notation to the game's standard rules. For example:

The cleric has an eight-sided die (d8) per level to determine how many hit points (q.v.) he or she has.

Kask worked at TSR and was heavily involved in the development of AD&D 1st edition. It's unclear whether Kask introduced the term to D&D, or its appearance in Dragon #7 represents that a tendency already in use internally at TSR. We also know that Gygax read Alarums #1, and wrote a letter which was published in the second issue, though TSR didn't adopt the dice syntax for another two years.

The "d" is usually pronounced "dee", though there is a regional variant which pronounces it "die" (see “Dee-twenty” and “die-twenty” pronunciations on RPG Stack Exchange). It's certainly an abbreviation for "die" or "dice".

It's unclear whether or not "d8" was intended to represent an extant spoken form (e.g. "die 8"). The RPG community in 1977 was fragmented, and connected largely by regional word-of-mouth and fan-typed newsletters like Alarums & Excursions. What I do know is that reading "d" as "die" was actually practiced by a certain subset of gamers. We see this in some much later self-published works by Len Lakofka, an RPG writer from Chicago who was also an active fanzine author.

However, Gary Gygax was also connected to Lakofka via many of the same fanzines and groups. We might infer from the fact that Tim Kask speaks it as "dee" in his videos, and no TSR book that I'm aware of wrote "die", that TSR did not interpret it that way. I suspect that "D" was an abbreviation necessitated by limited pagecount of fanzines, and that "die 8" was a reading some groups made from the abbreviation.

If you're interested in more specific detail as to which exact individual coined the "d" notation and the circumstances of came to enter D&D, you might leave a comment on the current latest video on Tim Kask's YouTube channel. He frequently answers viewer questions on such matters.

Addendum

Some commenters have suggested that the XdX notation comes from a need to avoid ambiguity in cases such as "20d6", which if written "20 6D" could be misheard in speech as "26D". I disagree with the notion that this was the origin of the notation, for the following reasons:

  1. The problem only happens when rolling exactly twenty, thirty, or another multiple of ten dice. "19 6D" and "21 6D" aren't ambiguous in the way "20 6D" is.
  2. Rolls as high as 20 dice were almost unheard of in June 1975 when Johnstone defined the syntax. The only D&D books published at this point were the original boxed set and possibly the first printing of Greyhawk, which was brand new at the time (source) and first introduced rules for magic users of up to 20th level who would be capable of using twenty dice. There were no rules for 30th level characters. It was also common for characters to retire around level 14 when their high power levels meant the game was no longer sufficiently challenging.
  3. If you thought someone did say "roll 26D", it wouldn't make any sense anyway, because all other die rolls specify the die type. Twenty-six of what type?
  4. XdX notation actually creates a more common problem in this regard. 4d6 can be misheard as "forty-six"; similarly 6d6 as 66, 7d6 as 76, 8d6 as 86, and 9d6 as 96. Rolling four dice is considerably more common in D&D than rolling 20 dice.
  5. Considering why Johnstone (or whoever may have coined the syntax before him) selected XDX rather than X XD would only be speculation. While can certainly debate which format is superior or more sensible, the main reason that we use XdX notation is simply that it was adopted by TSR beginning with the Advanced Dungeons & Dragons 1st edition Player's Handbook (1978), and this work was so influential that it became standard in roleplaying games.
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    MT0's answer seems to be the 2nd part of this. The "d" notation provided a compact way of saying how many dice. Your "3 6-sided dice" would be 3d6 (it was understood they were added together). A weapon might do 1d8 or 2d4 damage. A fireball did (level)d6 damage. A treasure could have 4d8 diamonds. And way back, popular rogue, nethack ... dungeon computer games borrowed it. Zweihander swords did 5d4! – Owen Reynolds Nov 28 '20 at 15:33
  • I'm not convinced by Addendum item 4. How often do you get a fourty something sided die? and how often would you roll that many die? I would think context would make it easy to separate 4d6 from 46. Either way, excellent answer and history lesson +1 – Fred Stark Nov 30 '20 at 2:50
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    @FredStark I mean that "4d6", pronounced "four dee six", can be misheard as the number "46", pronounced "forty-six". The reverse is also true. For example, if the DM says "you take 46 damage", this can easily be misheard as "you take 4d6 damage". – Quadratic Wizard Nov 30 '20 at 6:55
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    I'm not convinced by addendum 1. Should "1 2d" be 1d2 or d12 (where the kerning is slightly off in print)? Just because the ambiguity does not exist for certain numbers doesn't mean the ambiguity doesn't exist at all; as you said, "the problem only happens when ..." so there is a problem and using the D as the delimiter eliminates this ambiguity. It may create other issues (like hearing "4d6" as "fourty-six", which could be solved by saying "four-die-six") but that is beside the point as it eliminates the ambiguity of which digits are the number of dice vs. the number of faces on the dice. – MT0 Nov 30 '20 at 9:48
  • @QuadraticWizard In my experience (5e, Pathfinder as GM and as player) those misunderstandings are very rare. In most cases the listener knows if they're expecting to hear a number (46) or a set of dice to roll (4d6) – Carl Kevinson Nov 30 '20 at 15:20
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It is less confusing to say "twenty D six" (20d6) than to say "twenty six-D" (20 6d), which could be interpreted as a "twenty-six-D" (26d).

Having the D act as a delimiter between the quantity of dice and the number of faces of the dice makes it explicit which number applies to which part of the statement.

Then to get to "d6", that is just an abbreviation for "1d6" where the initial "1" is redundant information and can be dropped for simplicity.

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    This is the TLDR that should be at the top of Quadratic Wizard's answer. – Herohtar Nov 29 '20 at 5:35
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You can take a look to this article.

It seems that the "d" notation was introduced in order to remove the ambiguity of a mixed instructions (verbal and numerical).

The "d" notation introduces a dice-specific algebra that allows you to find easily the equivalence of dice roll. e.g.:

roll 30 times a d10 dice = 30d10 = 3d10×10 = take your 3 d10 dice and roll them 10 times

p.s.: I think the "d" stands for "dice" because a d6 has 3 spatial dimensions and only one numerical.

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    You seem to be using the word "times" inconsistently. First you write "roll 30 times" to mean 30 instances of rolling a die, then you write "roll 10 times" to mean rolling dice and then multiplying by 10. – Tanner Swett Nov 28 '20 at 0:49
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    @TannerSwett In the context of rolling multiple, you can also talk about "roll your 3 d10 dice 10 times and take the maximum" so it is not necessarily about multiplying, it can be that you want to roll multiple sets of 3d10 and do something with that set of 10 values you have generated. – MT0 Nov 28 '20 at 0:54
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    @TannerSwett I think you misread the post. It doesn't say multiply, it says roll ten times. The thing you are rolling is one set of three dice. To get 30 rolls, you can roll the three dice ten times. The number of rolls need not and often does not match the actual number of dice available to roll. – barbecue Nov 29 '20 at 22:27
  • @barbecue I think you're right; I was confused by both the notation and the wording and so I misunderstood what the post was saying. I'll submit an edit request to change the wording a little. – Tanner Swett Nov 30 '20 at 1:09
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The number comes after the 'd' since this is addressing a subtype. Consider the original sentence to be "a dice with 6 sides" rather than "a six sided dice" and it also makes sense from your language point of view. But the underlying thought here is that we want to address a number of dice of certain kind (six-sided ones). And in such type systems you typically start with the full type name with the major type (e.g. a 'd' for dice) and the less discriminatory information specifying the sub-type follows (e.g. 6 for six-sided dice). That way your brain can narrow down on the general type of things (aha! a dice! let's grab the dice box!) and then is "led" to the more specific subversion (oho, so only the 6 sided ones).

Product model names often follow the same principle: You get a Windows 10 and not a 10 Windows. You get a Tesla S and you get AMD Ryzen 5 3600 or 3 AMD Ryzen 5 3600 if you get multiple ones.

Obviously not everything is used as a way to type things and make up categories of stuff. Sometimes we give them just random names/identifiers. Often we just use descriptive language to identify products and then this does not hold as you are not using a type reference but you are describing the thing you want ("a green Pepper" is not a type shortcut, it just describes what happens if Ironman paints his girlfriend green, i.e. it's not a type based name for a group of things).

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While die may be the term for a single dice when you say "6-die" it sounds like you are talking about more than a single one. calling a die a "D6" or "D20" is more convenient overall and also easier to say how many of each type you want. "6 D6 and 2 D20" as an example. If you where to say it as "6 6-die and 2 20-die" it would be more confusing.

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  • It seems to me that most of the problem you’re talking about comes from the pluralization being nonstandard. (Is there any other word that becomes “ice” when pluralized from “ie”?) You’d say “6 6-dice”, just as you’d say “6 6-tuples”. – user34579 Nov 28 '20 at 0:52
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    @Jessy It isn't the pluralization or lack of pluralization I was referring to but the problem of saying numbers in a sequence and the possibility of confusing which ones stand for the number of dice and which stands for the type of dice. By using a standard of d6/10/d20 it makes it very clear how many dice and of what type. This problem is mentioned in other answers. – Joe W Nov 28 '20 at 3:37