Choose a school of magic to which you already have applied the Spell Focus feat. Your spells of that school are now even more potent than before.
Prerequisite: Spell Focus (chosen school).
Benefit: Add +1 to the Difficulty Class for all saving throws against spells from the school of magic you select. This bonus stacks with the bonus from Spell Focus.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic to which you already have applied the Spell Focus feat.
Source: Player’s Handbook (version 3.5), page 94.