Talk:Armor class

THAC0
'''Here is the information saved from the THAC0 article. We should add some of it here.'''

THAC0 is an acronym constructed from the phrase "To Hit Armor Class 0" which comes from the role-playing game Dungeons & Dragons. THAC0 was first introduced in the Dungeon Master's Guide of the 1st edition of Advanced Dungeon & Dragons(1979) as a convenient way to calculate the entries of the combat tables that were used to determine the success of a physical attack. In the 2nd edition of Advanced Dungeons & Dragons THAC0 replaced the combat tables completely and therefore most people associate THAC0 closely with this edition of the AD&D game.

THAC0 is a measure of a character's or monster's ability to successfully attack an opponent in physical combat. Each creature has an armor class that determines how hard it is to hit that creature, with lower numbers representing higher difficulty. For example, a normal, unarmored human has an armor class of 10 while a heavily armored fighter might have an armor class of 1 or 0.

The success of a character's or monster's attack is determined by rolling a 20-sided die. If the number obtained equals or exceeds the attacker's THAC0, the attacker has successfully hit a target with armor class 0. If the target has an armor class different from zero, the target's armor class is subtracted from the attacker's THAC0, and that number is what the attacker's roll must equal or exceed.

A simple formula for quickly calculating the success or failure of an attack roll using THAC0 is:

THAC0 - (roll on a d20) = AC Hit.

For example, if the attacker has a THAC0 of 17 and rolls a 15 on his or her To Hit roll, the attacker has succeeded in hitting if the target's armor class is 2 or higher (2 or worse).

High THAC0s signify that an attacker is poor at hitting targets, while low THAC0s indicate the attacker can hit targets with ease. There is nothing particularly significant about an armor class of 0 other than the fact that it simplifies calculations.

The idea that a creature is harder to hit when its armor class goes down was felt by some players to be counterintuitive. Furthermore it produces mathematical inconsistencies in the game system with magical armours as a beneficial magical modifier is always denoted with a "+" although it has actually to be subtracted from the AC. For example an ordinary chain mail armour provides a character with an AC of 5 whereas a magically enhanced version with a +3 modifier would improve the AC to 2 and not 8 as one might expect.

In the 3rd edition of D&D (2000), the armor class system was changed and THAC0 is no longer used in the following D&D editions. THAC0 is still sometimes unofficially used with the HackMaster RPG, which is derived from 1st and 2nd edition AD&D.

While some players mourned the loss of the THAC0 system in the change to 3rd Edition D&D, many invited the new system, which replaces THAC0 with a bonus to the attack roll called base attack bonus (in short BAB) and features an armor class rating that increases as it becomes harder to hit a creature. However, mathematically both systems are equivalent and the THAC0 and AC values of AD&D can be easily transformed to the AC and base attack bonus values of the later edition.


 * BAB = 20 - THAC0
 * THAC0 = 20 - BAB
 * 3rd Edition Armor Class = 20 - 2nd Edition Armor Class
 * 2nd Edition Armor Class = 20 - 3rd Edition Armor Class

Web Warlock (talk) 00:49, 1 March 2008 (UTC)


 * yeah something definately needs to be added telling what THAC0 stands for. wish i hadn't missed the AfD for THAC0 but it does need to be explained in some way as for many years it was a crucial part of D&D and a turning point for some to switch to 3rd edition and leave older editions behind. the controversies alone should have been worthy of it having its own article rather than implying it was a "game guide" as the deletion was closed as. but other than a bit about how it changed gameplay and what it stands for what does the armos class article need of whats left of the THAC0 article? shadzar|Talk|contribs 18:06, 1 March 2008 (UTC)

This would probably count as "original research", but I'll note that while the AD&D DMG lists "To Hit A.C. 0" in its monster tables, it does so with that phrase, and never uses the abbreviation THAC0. The first product I know of to actually explain how to use THAC0 instead of the to-hit tables is the 2e Players Handbook... published in 1989, ten years after the DMG. Some products in between listed THAC0 for pre-generated PCs, NPCs, and/or monsters, but since the rules did not explain what it was or how to use it, it remained a mystery to many players until 2e. (In the B/X and BECMI lines, THAC0 was not introduced until the Rules Compendium, released in 1991.) — Preceding unsigned comment added by 2601:4C0:4002:52E0:28CA:86A3:D00F:6069 (talk) 19:29, 16 March 2024 (UTC)


 * I second this. As the assertion above that it appeared in the 1E DMG is at best misleading, if not actually properly untrue, it is causing people to misunderstand how the classic game actually worked.  I don't believe Wikipedia intends articles to be factually misleading, but in my opinion this one is. SolomoriahBFRPG (talk) 15:41, 17 April 2024 (UTC)

Relevance FTW!
Uhm. *Gently points you all in the direction of the reference* I'ma gonna take that off, as y'know, Gary Gygax, as saddening though his death may have been, STILL can't be put as any sort of D&D related reference. Yet, of course. --  Scar  † Contributions  † 09:42, 1 August 2008 (UTC)
 * Well, guess who feels dumb. I forgot how to add the box that says the article needs references.  Please smart people, do away with my sins!  --  Scar  †  Contributions  † 09:46, 1 August 2008 (UTC)

I haven't played (A)D&D in roughly 30 years and certainly don't own any manuals to reference any more, but in the original boxed set and in The Monster Manual which was the first volume of AD&D, the highest/worst AC was 9. AC10 was introduced in The Players' Handbook, which was the second AD&D volume. Basic D&D may have used AC10; I never played that variant.

Tsgsh (talk) 21:53, 21 June 2011 (UTC)


 * Basic D&D used AC 9 as the "worst" AC; consistent through Holmes, Moldvay, and Mentzer editions, and I think RC but I do not have a reference for that. SolomoriahBFRPG (talk) 15:39, 17 April 2024 (UTC)