This optional system allows a character to trade out half her feats in order to gain the benefits of a secondary class. These rules enable characters to gain many of the benefits of multiclassing without sacrificing advancement in their primary classes, and creates opportunities to explore novel character concepts, such as a barbarian whose rage stems from being afflicted by the gods with an oracle’s curse and revelations.

Under the standard rules, multiclassing can lead to a wide disparity in character ability. With this system, each character can choose a secondary class at 1st level that she trains in throughout her career, without giving up levels in her primary class. Once selected, this choice is permanent (though if using the retraining rules, the secondary class can be retrained by paying half the cost of retraining all her class levels). A character who selects this option doesn’t gain feats at 3rd, 7th, 11th, 15th, and 19th levels, but instead gains class features from her secondary class as described on Table: Multiclass Character Advancement. It is probably a good idea to use either this variant system or normal multiclassing, but it’s possible for the two systems to be used together. In a game using both systems, a character can’t take levels in the secondary class she gains from this variant.

See the PFSRD page for more details.

Multiclass Character Advancement Table