View source for Spellcasting

The number of spells of each level which a Cleric or Magic-User may cast per day is shown on the appropriate table in the Characters section, above. Each day, usually in the morning, spellcasters prepare spells to replace those they have used. Clerics do this through prayer, while Magic-Users must study their spellbooks. Spells prepared but not used persist from day to day; only those actually cast must be replaced. A spellcaster may always choose to dismiss a prepared spell (without casting it) in order to prepare a different spell of that level.

Spellcasters must have at least one hand free, and be able to speak, in order to cast spells; thus, binding and gagging a spellcaster is an effective means of preventing him or her from casting spells. In combat, casting a spell usually takes the same time as making an attack. If a spellcaster is attacked (even if not hit) or must make a saving throw(whether  successful or not) on the Initiative number on which he or she is casting a spell, the spell is spoiled and lost. As a specific exception, two spell casters releasing their spells at each other on the same Initiative number will both succeed in their casting; one caster may disrupt another with a spell only if he or she has a better Initiative, and chooses to delay casting the spell until right before the other caster.

Some spells are reversible; such spells are shown with an asterisk after the name.