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In this high-powered campaign variant, characters essentially take two classes at every level, taking the best aspects of each. The process is similar to multiclassing, except that characters gain the full benefits of each class at each level. If the two classes you choose have aspects that overlap (such as Hit Dice, attack progression, saves, and class features common to more than one class), you choose the better aspect. The gestalt character retains all aspects that don't overlap.
The gestalt character variant is especially effective if you have three or fewer players in your D&D group, or if your players enjoy multiclassing and want characters with truly prodigious powers. This variant works only if every PC in the campaign uses it, and it results in complicated characters who may overwhelm newer D&D players with an abundance of options.