Base saves are ratings of how capable a creature is at (partially) avoiding certain forms of attack on the basis of classes obtained. To these are added bonuses (e.g. from feats and abilities) to get a creature's saving throws.
Base saves are determined by different methods in the pre-epics (character level 20 and below) and in the epic levels (character level 21 and up). In the pre-epics, the three ratings (fortitude, reflex, and will) are determined by class level. The ratings for each of a character's classes are added together to get the character's base saves. Some classes are better at one type of save than others (as described by that class' primary saving throws), which can result in an uneven progression in the base saves. Specifically, primary saving throws (high saves) go up +2 at class level 1 and +1 at each even class level, while other saving throws (low saves) go up +1 every third class level.
For example, a level 6 barbarian / level 4 rogue has a fortitude save of 5 from the barbarian levels (high save) and 1 from the rogue levels (low save), resulting in a base fortitude save of 6. This character has a reflex save of 2 from the barbarian levels (low save) and 4 from the rogue levels (high save), resulting in a base reflex save of 6. The will save is 2 from barbarian levels (low save) and 1 from rogue levels (low save), resulting in a base will save of 3.
After a character reaches (character) level 20, his base saves cease being affected by which classes advance in level. Instead, all three base saves increase +1 at each even character level (starting at level 22). Thus, an epic character's base saves are determined by both his character level and his class levels at level 20. (This is similar to how the base attack bonus is calculated for epic characters. See also epic progression.)
|Epic base save||+1||+2||+3||+4||+5||+6||+7||+8||+9||+10|
For example, consider a level 16 barbarian / level 14 rogue who had a 12/8 class split at level 20. This character's base fortitude save from pre-epic levels is 10 (8 from twelve barbarian levels and 2 from eight rogue levels). To this is added 5 for his epic levels (character level 30), resulting in a base fortitude save of 15.
A character with multiple classes can benefit from higher base saves than a single-classed character. This is because of the +2 to primary saving throws at class level 1. While the base saves of a single-classed character can go up to at most +12 at level 20, the base saves of a character with two classes can go up to +14 (for example, the base fortitude save of a 10/10 fighter/barbarian), and the base saves of a character with three classes can go up to +16 (for example, the base fortitude save of a 8/6/6 fighter/barbarian/ranger).
Multiclassing can also help mitigate a character's lower saves by matching up classes with different primary saving throws. This is particularly of note since most base classes have only one primary saving throw, resulting in one base save of 12 and two of 6 if that class is the only one taken through level 20. On the other hand, an 8/6/6 fighter/wizard/rogue has base saves of 10 (fortitude), 9 (reflex), and 9 (will) — a lower highest save, but improved lower saves.
To receive the base save benefits of multiclassing, the additional classes must be taken prior to character level 21.