To make an attack roll, the character's THAC0 must be known. This depends on
the group and level, if the attacker is a player character or NPC, or the Hit
Dice if the attacker is a monster or an animal. All 1st-level characters have
THAC0s of 20, regardless of class.
For a character of level 1 through level 20, consult Table 53. This table lists the THAC0 number of each group through 20th level, so
players don't have to perform any calculations.
For a character higher than 20th level, find the Improvement Rate for the
character's group in Table 54. There you'll find the number of levels a character must advance to reduce
his THAC0 by 1 (or more) points. Calculate the character's THAC0 according to his
The DMG contains the information on monster THAC0s.
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