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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 level.

The (DMG) contains the information on monster THAC0s.

Table 53: Calculated THAC0s

Level
Group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Priest 20 20 20 18 18 18 16 16 16 14 14 14 12 12 12 10 10 10 8 8
Rogue 20 20 19 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11
Warrior 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
Wizard 20 20 20 19 19 19 18 18 18 17 17 17 16 16 16 15 15 15 14 14

Table 54: THAC0 Advancement

Group Improvement Rate
Points/Level
Priest  23
Rogue  12
Warrior 1/1
Wizard  13
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