Old versions of D&D use Vancian magic, often characterized as “memorize and forget.” This was widely disliked by players, not only because it seems unrealistic, but because it imposed far too much restriction on spell choice. As a result, players tended to arm a narrow selection of practical offensive or healing spells, not daring to waste slots on more interesting spells.
Some gaming groups adopted house rules dispensing with Vancian magic, replacing it with spell points or allowing spells to be selected at will. This ran the danger of making spellcasters too powerful.
The house rule proposed below is designed to preserve most of the game balancing properties of Vancian magic, while allowing some flexibility in spell choice at the lower levels.
At the start of each day, after a full night’s rest, the spellcaster summons his magical energies and arms a certain number of spells of each level for the day, just as in the official rules.
The spellcaster is allowed, however, to swap a spell for another of the same level, spending one non-combat turn to do so. The downside is that there is a chance that the spell, when cast, will have no effect (prior to any applicable saving throw).
Spell Level Chance of Failure
1 10%
2 20%
3 30%
4 40%
Up to 2 spells of each level may be swapped per day. This applies to both clerics and magic-users. Spells that are 5th Level or above may never be swapped.
This rule gives low-level spellcasting greater flexibility, encouraging more creative choices, without making high-level spellcasting unduly powerful. There is still value in choosing spells wisely at the start of the day, but this is no longer absolute.
Note that the distinction between clerics and magic-users is maintained, since clerics may choose from any spells in a level, while magic-users may choose only from among spells in their book.
For those who would like an in-universe fluff rationale for this mechanic, consider the following. A night’s rest recharges the spellcaster’s magical energies. At the start of the day, the spellcaster summons these energies (by reading from a magic user’s spellbook or by prayer for a cleric) and then arms a selection of spells, concentrating on each spell and associating a triggering magical phrase with that spell (using a different phrase for each iteration of a spell). Each of these prepared spells can be cast by merely uttering the magical phrase (along with any physical requirements specific to that spell).
“Swapping” a spell requires the caster to take some time to concentrate, associating one of the triggering phrases with a different spell than originally intended. Since the phrase may not be as compatible with the newly chosen spell, and because the caster’s magical energies are no longer at their peak, this improvised arming is less effective, especially for the more powerful spells, which require greater mental energies. Spells of the 5th Level and above require so much energy as to make them impossible to prepare except at peak performance in the morning.