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As important as your character's race is, it's (usually) a one-time choice made at the beginning of your character's career. Your selection of class levels, on the other hand, is a recurring decision that is shaped both by personal preference and the realities of the campaign world.
The base character classes in the d20 System cover many of the basic fantasy archetypes, from the raging warrior to the crafty wizard. But the number of character roles imaginable is essentially infinite, and even when factoring in all the prestige class choices available, you may not be able to find the combination of class features that perfectly captures your mental image of your character.
This section presents a wide variety of options that allow you to tweak the existing character classes without completely scrapping them and starting from scratch. Whether they involve swapping out one class feature for another or trying a new twist on an old favorite, these variants allow the players and the DM to retain the familiarity of existing classes while exploring worlds of difference.
With your DM's permission, you can use any one of these variant classes in place of the standard class of the same name. Depending on the campaign world, variant classes may exist side by side with standard classes, or they may replace standard classes entirely. For instance, the DM may decide that all monks must choose a fighting style that reflects their original training, or that all barbarians must choose a totem.
For those who prefer a less specialized approach to character building, the section also includes a trio of "generic" classes that allow players the ability to custom-craft their character's unique identity.
Throughout this section, the term "character class" is used to refer to any class that may be taken by a 1st-level starting character. This includes the classes that appear in the d20 System, as well as any similar class in other products.