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	—
Cleric or Druid	20	20	19	18	18	17	16	16	15	14	14	13	12