Calculating THAC0 (DMG)

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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.
For a character of level 1 through level 20, consult Table 38. 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 37. 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. A priest, for example, improves by two for every three levels he advances. A 5th level cleric would have a THAC0 of 18. A rogue (a thief or bard) improves one point every two levels. A 17th-level rogue would have a THAC0 of 12. Table 38 lists the THAC0 number of each group at each level.
Creatures do not have character classes and levels, so they calculate THAC0s differently, basing it on the Hit Dice of the creature. Table 39 lists the THAC0 number for monsters having various numbers of Hit Dice. When a creature has three or more points added to its Hit Dice, count another die when consulting the table.
 Table 37: THAC0 Advancement
Group 	Improvement Rate Points/Level
Priest 	
Rogue 	
Warrior	1/1
Wizard 	
 Table 38: Calculated THAC0S
 	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	Level	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 39: Creature THAC0
 	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice	Hit Dice
or less	1-1 	1+ 	2+ 	3+ 	4+ 	5+ 	6+ 	7+ 	8+ 	9+ 	10+ 	11+ 	12+ 	13+ 	14+ 	15+ 	16+
20 	20 	19 	19 	17 	17 	15 	15 	13 	13 	11 	11 	9 	9 	7 	7 	5 	5