Casting: in casting a spell, the three mental ability characteristics are applied using a common formula:

• .d20 + Referenced Skill Ranks + Ability Score Modifier – Spell Level
  
  

  Spell Type	Caster Type	Skill Referenced
  Arcane	Mage	Spellcraft
  Base Score	Sorceror	Concentration
  Base Score	Bard	Perform
  Divine	Cleric	Knowl. Religion
  Base Score	Paladin	Knowl. Religion
  Base Score	Druid/Ranger	Nature Lore
  Psionic	Psion/Psychic Warrior	Psicraft

  Counterspelling: In order to counterspell, you must first know the spell. When you elect to counterspell, you must have a spell of the same school memorized.

  Summoning: Spells of the summoning school have an extended duration should the caster forego further spellcasting and spend his magical efforts on maintaining the spell. The duration that the spell is maintained is based on the continued checks of the caster. Should the caster’s concentration be broken, control ends immediately.

  Sub-School	Skill Check	Ability Score	DC to Maintain
  Creation	Concentration	WIS	Spell Level + Duration
  Summoning	Intimidation	CHA	Spell Level + Duration + 1/2 Creature HD
  Calling	Diplomacy	CHA	Spell Level + Duration + Creature HD

  Should control end before the normal duration of the spell, the creature remains for the duration of the spell. If Intimidation was used, the creature attacks the caster. If Diplomacy was used, the creature is free-willed and reacts based upon an encounter reaction check.

  Illusion: Illusions gain a difficulty class to determine if they are believed. The difficulty to discern an illusion is equal to a Sense Motive check modified by wisdom. The difficulty class of the spell is equal to the caster’s Spellcraft check modified by intelligence. Each round after the first, the illusion loses one point from its difficulty class.