An acronym for "To Hit Armor Class Zero" with the number '0' standing in for the final letter. An easily memorized system in which the requirement to hit the armor class of the target is subtracted from the standing number possessed by the combatant necessary to hit AC 0. See the table below for the THACO that combatants possess according to their class and level:

Thaco Redux.jpg

All persons with combat experience, who do not possess a level, with the exception of mages, illusionists, thieves and assassins, have a THAC0 of 20, equal to a 1st level fighter.

Suppose an 8th level thief (THAC0 of 18) attempts to attack a creature with an AC of 7. The chance to hit is easy to calculate: it is 18 - 7 = 11. If the thief rolls an adjusted 11 when attempting to hit, the thief will hit. See Roll to Hit.

See Attacking