From the introduction: D20 Unchained is a rules module that can be used for any D20 game such as Pathfinder or Dungeons & Dragons 3rd edition. It removes classes and levels from the player characters, no longer are players required to track experience or calculating multi'class builds to achieve a character concept. Characters grow and develop as the player wishes, with minimal restrictions. D20 Unchained is designed so that 95% of the current rules system in use is still used, there is no requirement for re-writes of rulebooks, monsters or adventure modules as all of the changes are on the player side of the table. Depending upon the skills chosen, starting characters may be somewhat sturdier, but with less skills overall. D20 Unchained was designed using the D&D 3.0 edition rules.