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 D20
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This article is a D20 reference page
The System Reference Document is a comprehensive toolbox consisting of rules, races, classes, feats, skills, various systems, spells, magic items, and monsters compatible with the d20 System version of Dungeons & Dragons and various other roleplaying games from Wizards of the Coast.
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Hit Dice
This line gives the creature’s number and type of Hit Dice, and lists any bonus hit points. A parenthetical note gives the average hit points for a creature of the indicated number of Hit Dice. A creature’s Hit Dice total is also treated as its level for determining how spells affect the creature, its rate of natural healing, and its maximum ranks in a skill.
System Reference Document -> Reading Creature Entries (SRD)