THAC0 is an acronym for To Hit Armor Class 0 (Zero), it represents how likely a creature's physical attacks may hit the target. The lower the number the better.
How it works Edit
A 20-sided icosahedral die is rolled every time by the creature who's attempting a physical attack, and if the roll number is equal or higher than the subtraction of attacker's THAC0 from target's Armor Class value, the hit succeeds; if lower, the attack misses.
By twisting the above equation, we can also get the minimum roll number required to hit:
Minimum roll required to hit = (attacker's THAC0) - (target's base AC + target's AC modifier)
For example, a THAC0 of 17 means to hit a target with an AC of -1, the die roll must be at least 17-(-1)=18, a 3/20 or 15% chance of success; while a much lower THAC0 of 3 means that to hit the same target, the die roll must be at least 3-(-1)=4, a 17/20 or 85% chance of success.
After enabling the related FEEDBACK settings in GAMEPLAY option menu, the in-game feedback messages will show as:
(Attacker's roll) + [(target's AC modifier) - (attacker's THAC0)] = result
With the result number plus target' 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're some items, a HLA, and the proficiency points spent on certain fighting styles that boost the critical hit rate.
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.
BGWiki writing formats Edit
If a creature with a THAC0 of 6 wields a weapon with THAC0: +2 in its statistics 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.