This archetype transforms a prepared spellcaster into a spontaneous spellcaster.  For example, instead of a wizard preparing spells from his or her spellbook each morning, that wizard might spontaneously cast spells as a sorcerer, instead.  Generally, this is sufficient to decrease your tier by 0.5 (move the class half a tier lower).

Like other spellcasters, a character using this archetype can cast a certain number of spells per day. His base daily spell allotment is given on Table: Spontaneous Spells per Day.  In addition, he receives bonus spells per day if he has a high ability score (see Table: Ability Modifiers and Bonus Spells).  Use the same ability score which would govern bonus spells per day for the base class.  For instance, a 1st-level druid with 18 Wisdom using this system can cast three 0-level spells and two 1st-level spells per day.

However, the spellcaster's selection of spells known is limited. At 1st level, the character begins play knowing four 0-level spells and two 1st-level spells of your choice, plus a 1st-level domain spell (if a cleric), a school spell (if a specialist wizard), or any other extra spell slots given by the base class. At each new level in the character's spellcasting class, he gains one or more new spells, as indicated on Table: Spontaneous Spells Known.

Characters who use this option lose the ability to spontaneously cast cure, inflict, or summon nature's ally spells in place of other spells. However, each time the character gains a new spell level, he gains one or more bonus spells known to add to his list. A cleric may add his two domain spells to his list of spells known, while a druid may add the appropriate summon nature's ally spell to her list of spells known. (An entry of 0 on the table indicates that the cleric knows only his domain spells of that level, and the druid knows only the summon nature's ally spell of that level.)  This follows simiarly for other base classes with specific spontaneous spellcasting options.

If a prepared spellcaster would depend upon a spellbook, familiar, or other resource to prepare spells each day, he or she no longer requires that resource (but they can still benefit from options for that resource in other ways, such as a witch's familar serving as a scout, etc.).  Fifteen minutes of concentration is sufficient to ready spells for the day after a full night's rest, as a Sorcerer readying spells.

Upon reaching 4th level, and at every even-numbered class level after that, a spellcaster can choose to learn a new spell in place of one he already knows. This functions identically to the sorcerer's ability to swap out known spells, except that a cleric may never choose to lose a domain spell, a druid may never choose to lose a summon nature's ally spell, etc.

For example, a cleric has chosen the domains of Good and Healing, which means that at 1st level, he automatically knows cure light wounds and protection from evil. In addition, he chooses four spells from the list of 0-level cleric spells (cure minor wounds, detect magic, light, and read magic) and two spells from the list of 1st-level cleric spells (bless and shield of faith). He now knows four 0-level spells and four 1st-level spells.

Another example: At 4th level, a druid learns a new 0-level spell and a new 2nd-level spell. She can also choose to replace one of her 0-level spells known with a different spell of the same level. She chooses to replace know direction with detect poison.

This archetype modifies the Spells, Domains, Spontaneous Casting, Arcane Schools, and similar class features of the base class.  It is still combatible with other archetypes which modify spellcasting, so long as those archetypes do not explicitly reference prepared spellcasting or prepared spell slots.