THAC0 is an acronym for To Hit Armor Class 0 (Zero). It is a number that represents the chances that a physical attack hit its target. Lower numbers are better.
How it works Edit
Every time a creature makes a physical attack the game simulates an attack roll, which is the roll of a 20-sided die. The attack is successful if the number rolled is equal or greater than the to-hit number, otherwise it misses. The to-hit number is obtained by subtracting the target's Armor Class from the attacker's THAC0.
To-hit number = THAC0 - AC
For example, a character with a THAC0 of 17 trying to hit a target with an AC of 1 needs a roll of at least 17-1=16 to succeed. This is a chance of 5/20, or 25%. A character with a much lower THAC0 of 5 trying to hit the same target needs a roll of at least 5-1=4. This is a chance of 17/20, or 85%.
The game has a option that adds attack rolls to the feedback screen. This option is located in the Options menu, Gameplay section, Feedback subsection. When this setting is enabled, the feedback screen displays the attack rolls in this format:
(Attacker's roll) + [(target's AC modifier) - (attacker's THAC0)] = result
With the result number plus target's effective AC, which is not shown, greater than or equal to zero will be a hit.
THAC0 can be modified by many permanent or situational variables, such as the creature's class(es) advancement, weapon proficiency, strength or dexterity adjustments based on the wielding weapon class, and the changes from items and spells.
There're four different THAC0 progression rate by class level:
- Warrior group (Fighter, Ranger, Paladin) and Monk have the best THAC0 progression, with a THAC0 that decreases at a rate of 1:1, to a max of base 0 at level 21.
- Priest group (Cleric, Druid, Shaman) except Monk have the second best rate of 2:3, to a max of base 6 at level 22.
- Rogue group (Thief, Bard) have a rate of 1:2, to a max of base 10 at level 21. In addition to the mathematical oddity of having a better THAC0 than a priest at level 3, rogues remain roughly on-par with priests (albeit slightly in the priest's favor) until level 9, and are thereafter surpassed by priest classes at an increasing pace.
- Wizard group (Mage, Sorcerer) have the worst THAC0 progression, at a rate of 1:3, to a max of 13 at level 22.
Regardless of a creature's THAC0, in Second Edition Dungeons & Dragons, a roll of 20 is always a critical (guaranteed) hit while a 1 is always a critical miss, both are 5% chances. There are some items, a HLA, and the proficiency points spent on certain fighting styles that boost the critical hit rate.
The previous progression rate gives the following table for THAC0 progression (note, that there are no further changes from level 22 on):
BGWiki writing formats Edit
If a creature with a THAC0 of 6 wields a weapon with THAC0: +2 in its in-game description, the adjusted value will be 6-2=4, for this reason, weapons' THAC0: +n and all kinds of THAC0 or to-hit bonuses from any other sources will be written as -n bonus, where n is the positive integer, and vice versa.
- In Dungeons & Dragons 3rd Edition and later, the to-hit system has been reworked to always trend upward by using attack roll + attack bonus - enemy AC ≥ 0 to land hits, where better scores don't go into the negative numbers but rather continue growing in the positive integers.