Thursday, January 28, 2010

Expanding James Maliszewski's "To Roll 20" D&D Combat System



Over at his absolutely must read "in praise of Old School RPG gaming" site Grognardia today, James Maliszewski takes on one of the typical myths regarding the complexity of the original AD&D and D&D games. Many people believe that you absolutely need the combat matrices on pages 74 and 75 of the Dungeon Master's Guide in order to run combat, and that continually looking at those charts can diminish the verisimilitude (to use a Gygaxian term) of the role playing experience.

In response to this criticism, James -- who has worked on the excellent "new school" games Colonial Gothic and Thousand Suns -- shares the chartless system he uses in his home campaign Dwimmermount. He describes the system as follows:

when a monster attacks, I roll a D20, and add the monster's Hit Dice and the target's (descending) armor class to the result of the dice roll. If the sum is 20 or more, the attack is successful. This system is simple and quick and I don't need to consult any charts.


What James has done here, and it is mildly ingenious, is to deconstruct the old THACO system that was introduced late in the 1st edition. Essentially, under the THACO system each player wrote down a number that represented how high that player needed to roll on a d20 (after modifiers were added) for their character to hit Armor Class (AC) zero -- THACO stood for To Hit AC 0. Using the old THACO system, the player essentially ran the following subroutine:

  1. Roll d20 + Attribute Bonuses + Item Bonuses
  2. Get total.
  3. Subtract total from THACO.
  4. Result is AC you hit


The subroutine created what can only be described as a seesawing of arithmetic. First you add and get a result, then you subtract, and finally you compare that to a target number -- your THACO. It was a clumsy system, but it was better than the charts and became the basis for the 2nd edition combat system.

James has taken that seesawing subroutine and made it a one sided equation. It's fairly elegant.

  1. Roll d20 + Attribute Bonuses + Item Bonuses + Opponents AC
  2. Compare result to Target Number of 20


The system has the same mathematical effect as the THACO system, but adds a layer of elegance by putting all the arithmetic at the beginning of the process -- a negative AC would be a negative modifier to the initial roll.

On his site, James has included the chart for Fighting Men that he uses in his campaign. I don't know what chart James used to base his chart on, but I have calculated the "To Roll 20" bonuses for the character classes based on the charts in the old DMG.






I would like to mention one small thing when using these charts. James' "To Roll 20" system does make it slightly more difficult to hit certain armor classes than the charts on page 74 would normally be. For example:

Kin Rathslayer is a 7th Level Fighter with a +1 Longsword and a 17 Strength. Due to his weapon and Strength, Kin gets a +2 total attribute and item bonus to hit. He decides to attack Theodore Dudek "villainous rogue" who has an Armor Class of -8 due to equipment, attributes, and armor.

Using the chart on page 74 of the DMG, Kin would need to roll a 20 -- excluding his +2 bonuses -- to hit Theodore. With his bonuses, Kin needs to roll an 18. Kin has a 15% chance to hit Theodore and take the crown of "King of RPGs."

Using the "To Roll 20" system, Kin would roll d20 +6 (level bonus) +1 (weapon bonus) + 1 (strength bonus) - 8 for Theodore's AC. Kin needs to roll a natural 20 in order to hit Theodore. He has only a 5% chance of success.


This slight drawback occurs in the regular THACO system as well, and is due to the fact that on the charts each character class can hit multiple ACs on a roll of 20. For example, a 7th level fighter hits AC -6 through -10 with a total of 20. All four of those ACs have an equal chance of being hit by the fighter. A 4th level fighter hits AC -2 through -7 on a total of 20 on the DMG chart on page 74. The THACO and "To Roll 20" systems remove this long tail effect and substitute a much needed ease of play to the system. The statistics work out differently than the charts -- more for the non-fighter classes than for the fighter -- but the dividends in ease of play more than make up for that drawback.

Were I to run a 1st edition game, I would certainly use James' "To Roll 20" system and mock those who think that AD&D requires charts to determine if you hit your opponent. At least I would if I could figure out how the initiative rules actually work when using speed factors and weapon sizes.
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