|Treat bards as thief; paladins and rangers as fighters; and monks as clerics|
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 in Combat