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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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Space/Reach
This line describes how much space the creature takes up on the battle grid and thereby needs to fight effectively, as well as how close it has to be to threaten an opponent. The number before the slash is the creature’s space, or how many feet one side of the creature occupies. The number after the slash is the creature’s natural reach. If the creature has exceptional reach due to a weapon, tentacle, or the like, the extended reach and its source are noted in parentheses at the end of the line.
System Reference Document -> Reading Creature Entries (SRD)