THAC0

An acronym for To Hit Armour Class Zero with the number "0" used as the final letter. The number required to hit a zero armour class (AC) on a d20 is used to count backwards as the number for AC increases. For example, a combatant needing a 17 to hit AC 0 would need a 16 to hit AC 1, a 15 to hit AC 2, a 14 to hit AC 3 and so on. The number to hit equals the target's THAC0 minus the target's AC.

Attacker's Level/Hit Dice

Attacker	1st	2nd	3rd	4th	5th	6th	7th	8th	9th	10th	11th	12th	13th	14th	15th	16th+
Assassin	20	19	19	18	17	16	15	15	14	13	12	11	11	10	10	—
Bard	21	20	20	20	19	18	18	17	17	16	16	15	15	14	14	13
Cleric	20	20	19	18	18	17	16	16	15	14	14	13	12	12	11	11
Druid	20	20	19	19	18	18	17	16	16	15	14	14	13	13	12	12
Fighter, Paladin or Ranger	20	19	18	17	16	15	14	13	12	12	11	10	10	9	8	8
Illusionist or Mage	21	21	20	20	20	19	19	18	18	17	17	16	16	15	15	14
Monk	20	19	19	18	17	17	16	15	14	13	12	12	11	10	10	9
Thief	21	21	20	20	19	19	18	18	17	16	16	15	14	14	13	13
Commoner	21	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
Monster or Combat trained	20	20	19	18	18	17	17	16	15	15	14	14	13	12	12	11

The table reflects the lack of weapons training that some classes have over others, even compared to monsters (who have no training unless they're also levelled). With humanoid monsters that have experience levels, use the number that is best between their hit die and level.

See Also,
Attacking in Combat
Combat