# Gary Gygax Explains Dice

As the DM, the tools of your trade are dice — platonic solid-shaped or just about any other sort. The random numbers you generate by rblling dice determine the results based on the probabilities determined herein or those you have set forth on your own. In case you are not familiar with probability curves, there are two types which are determined by your dice: linear (straight line),'which has equal probability of any given integer in the number group, and bell (ascending and descending line), which has greater probability towards the center of the group of numbers than at either end. The two curves are illustrated thus:

[Image of linear curve for 10-sided dice, with each number having a 10% chance of coming up.]

Linear probability develops a straight line of ascending probability when used as a cumulative probability as shown above.

Bell distribution, when used to delineate the probability of certain numbers appearing, develops o curved line like this:

[Image of a bell curve for a 3d6 roll, with 3 and 18 being the least probable and 8 through 12 being the most probable.]

A single die, or multiple.dice read in succession (such as three dice read as hundreds, tens and decimals) give linear probabilities. Two or more dice added together generate a bell-shaped probability curve.

Before any further discussion takes place, let us define the accepted abbreviations for the various dice. A die is symbolized by "d", and its number of sides is shown immediately thereafter. A six-sided die is therefore "d6", d8 is an eight-sided die, and so on.-Two four-sided dice are pressed by 2d4, five eight-sided dice are 5d8, etc. Any additions to or subtractions from the die or dice are expressed after the identification, thus: d8 8 means a linear number grouping between 9 and 16, while 3d6 -2 means a bell-shaped progression from 1 to 16, with the greatest probability group in the middle (8, 9). This latter progression has the same median numbers as 2d6, but it has higher and loyver ends and a greater probability of a median number than if 2dl2 were used. When percentage dice are to be used; this is indicated by d%.

The 64 can be used to generate 25% incremental probabilities, random numbers from 1 to 4, with 1 it generates a linear 2-5, etc. It can be used to get 1 or 2 (1 or 2 = 1, 3 or 4 = 2) or in conjunction with any other dice to get linear or bell-shaped probability curves. For example, 2d4 = 2-8, 3d4 ^ 3-12, d4 d6 = 2-10, d4 d20 (as dlO) = 2-M.'when rolled in conjunction with another die, the d4 can be used to determine linear number ranges twice that shown on the other die, thus: d4 reading 1 or 2 means that whatever is read on the other die is the number shown; but if the d4 reads 3 or 4, add the highest number on the second die to the number shown — so if d8 is the second die 1 to 16 can be generated if a dl 2 is used 1 to 24 can be generated. If a d20 is used either 1 -20 (assuming the use of a standard d20 which is numbered 0-9 twice without coloring one set of faces to indicate that those faces have 10 added to the number appearing) or 1-40 (assuming that one set of faces is colored) can be gotten by adding 0 if 1 or 2 is rolled on the d4 and 10 or 20 (depending on die type) if a 3 or 4 is rolled. Linear series above this are possible simply bv varying the meaning of the d4 number; 1 always means add 0, but 2 can be interpreted as add the value (highest number) of the second die, 3 can be twice value, and 4 can be thrice value. Thus, a d4 reading 4 in conjunction with a d8 (linear curve 1-32) would mean 24 d8, or 25-32.

What applies to d4 has similar application with regard to d6, d8, dl2, and d20. The d6 has 16 2/3% intervals, d8 has 12 1/2% intervals, and d20 can have (1-2 = 1,3-4 = 2,5-6 = 3), while 1 to 5 can be easily read from a d20 (1-2 = 1, 3-4 = 2,5-6 = 3,7-8 = 4,9-0 = 5).

The d20 is used often, both as dlO and d20. The bell-shaped probability curves typically range from 2-20 to 5-50, i.e., 2, 3, 4 or 5d20 added together. Also common is the reading as above with one decimal place added to the result to get 20-200, 30-300, etc. In the latter case, a roll of 3 on one die and 0 (read as JO) totals 13, plus one place, or 130.

Non-platonic solid-shaped dice are available in some places. The most common of these is a ten-sided die numbered 0-9. As with the d20, this con be used for many purposes, even replacing the d20 if a second die is used in conjunction to get 5% interval curves (1-20). Also, the die can give 0-9 linear curve random numbers, as the d20 can.

Other dice available are various forms of "averaging" dice. The most common of these has six faces which read: 2, 3, 3, 4, 4, 5. The median of the curve it generates is still 3.5, that of a normal d6, but the low and high numbers, 2 and 5, are only half as likely to appear as 3 or 4. There is a 33 1/3% chance for either of the two latter numbers to be rolled, so the probabilities of absolutely average rolls are far greater. Other such dice have zeros on them, several low numbers, and so on. These sorts of dice, along with poker dice, "put & take" dice, or any other sort can be added in order to give you more flexibility or changing probabilities in random selection or event interpretation. For example:

The author has a d6 with the following faces: SPADE, CLUB, CLUB, DIAMOND, DIAMOND, HEART. If, during an encounter, players meet a character whose reaction is uncertain, the card suit die is rolled in conjunction with 3d6. Black suits mean dislike, with the SPADE equalling hate, while red equals like, the HEART being great favor. The 3d6 give a bell-shaped probability curve of 3-18, with 9-12 being the mean spread. SPADE 18 means absolute and unchangeable hate, while HEART 18 indicates the opposite. CLUBS or DIAMONDS can be altered by discourse, rewards, etc. | Thus, CLUBS 12 could possibly be altered to CLUBS 3 by offer of a tribute or favor, CLUBS 3 changed to DIAMONDS 3 by a gift, etc.

In closing this discussion, simply keep in mind that the dice are your tools. Learn to use them properly, and they will serve you well.

## Notes:

Probability, percentages, averages, there's a lot of math in this introduction to the most important tool of the gamer.

What does it mean that the average role of a 1d6 is 3.5 but the probability of any number coming up is 16.7?

Folksonomies: games mathematics statistics probability dice percentages

Taxonomies:
/hobbies and interests/games/role playing games (0.440189)
/science/mathematics/algebra (0.437083)
/science/mathematics/statistics (0.354084)

Keywords:
dice (0.932360 (positive:0.010754)), probability (0.817577 (negative:-0.034764)), bell-shaped probability (0.795464 (neutral:0.000000)), bell-shaped probability curve (0.751489 (neutral:0.000000)), bell-shaped probability curves (0.741148 (neutral:0.000000)), die (0.738728 (negative:-0.403878)), number (0.659333 (negative:-0.119863)), linear curve (0.652511 (negative:-0.214489)), 10-sided dice (0.640317 (neutral:0.000000)), random numbers (0.636647 (positive:0.139123)), Non-platonic solid-shaped dice (0.615944 (positive:0.264199)), greater probability (0.609285 (positive:0.203108)), greatest probability group (0.594985 (positive:0.259914)), (straight line),'which has equal probability of any given integer in the number group, and bell (ascending and descending line (0.578605 (neutral:0.000000)), eight-sided dice (0.570664 (neutral:0.000000)), four-sided dice (0.568434 (negative:-0.395029)), percentage dice (0.568132 (neutral:0.000000)), various dice (0.564090 (neutral:0.000000)), Linear probability (0.562531 (neutral:0.000000)), 0-9 linear curve (0.562389 (negative:-0.219702)), linear number (0.560473 (negative:-0.360398)), equal probability (0.544728 (neutral:0.000000)), poker dice (0.544704 (positive:0.320405)), cumulative probability (0.544539 (neutral:0.000000)), highest number (0.536705 (neutral:0.000000)), SPADE equalling hate (0.532715 (negative:-0.561980)), absolutely average rolls (0.531212 (neutral:0.000000)), eight-sided die (0.529965 (negative:-0.634387)), d20 (0.529006 (negative:-0.249078)), single die (0.527600 (negative:-0.403841))

Entities:
bell:Person (0.711182 (neutral:0.000000)), 3%:Quantity (0.711182 (neutral:0.000000)), 10%:Quantity (0.711182 (neutral:0.000000)), 2-M:Quantity (0.711182 (neutral:0.000000)), 25%:Quantity (0.711182 (neutral:0.000000)), 2%:Quantity (0.711182 (neutral:0.000000)), 5%:Quantity (0.711182 (neutral:0.000000))

Concepts:
Dice (0.982286): dbpedia | freebase
Arithmetic mean (0.557124): dbpedia | freebase
Probability theory (0.389037): dbpedia | freebase | opencyc
Trapezohedron (0.387089): dbpedia | freebase
Number (0.380321): dbpedia | freebase
Pentagonal trapezohedron (0.363761): dbpedia | freebase
Randomness (0.361941): dbpedia | freebase
Mean (0.348883): dbpedia | freebase

Advanced Dungeons & Dragons, Dungeon Masters Guide
Books, Brochures, and Chapters>Book:  Gygax , Gary (1989), Advanced Dungeons & Dragons, Dungeon Masters Guide, Retrieved on 2013-04-13
• Source Material [books.google.com]
• Folksonomies: games