Spontaneous Casting
Spontaneous Casting is a game mechanic introduced in Dungeons & Dragons 3rd Edition as a mainstream alternative to the traditional Dungeons & Dragons mechanic of Vancian Casting. It can be most simply summarized as a cross between Vancian Casting and a mana system.
To go into detail, Vancian Casting requires a caster to individually memorize each spell they want to cast for a day. Once expended, those spells are gone until they can be memorized again - it's like depleting a quiver of arrows. This means that if you can take away a wizard's spellbook, they lose all ability to regain any spells they cast until they get their hands on it again. In contrast, Spontaneous Casting means that the spellcaster simply "knows" their spells, and can use their spells-per-day mechanic (called "Spell Slots") to fuel whichever spell they want as they want it. And when they run out of spell slots? A simple nap, and they're ready to go again!
Spontaneous casters, unlike Vancian casters, generally can't learn spells from external spell-slots. This means that a sorcerer will never have the breadth of utility options that a wizard will have, because their total spells known is smaller. But, potentially, a spontaneous caster will have more "encounter versatility", since a vancian caster is completely shafted if they are not given time to prepare their spells in advance, whilst a spontaneous caster can immediately use whichever of their spells known is necessary for the situation at hand.
This mechanic ultimately inspired the AEDU System of Dungeons & Dragons 4th Edition, and is the foundation of casting as a whole in Dungeons & Dragons 5th Edition; in 5e, the "Prepared Caster" classes have a Vancian-style ability to change their "spells known" during their down time, but maintain the Spontaneous caster's ability to cast whatever they want, so long as they have appropriate spells per day.
List of Spontaneous Caster Classes
- Bard: Was changed from Vancian Casting to this in 3rd edition.
- Oracle: A Pathfinder creation to create a "core" divine magic full caster class using Spontaneous Casting.
- Sorcerer: The original Spontaneous Caster, and the class for whom the mechanic was invented.