Fighter/Ranger/Paladin/Barbarian :Thac0 is lowered by 1 for every level attained
Thief/Bard :THAC0 is reduced by one for every two levels attained
Cleric/Druid:Thaco is redused by 2 for every 3 levels attained
Mage:THACO is reduced by one for every 3 levels