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Semi-Conservative Spellcasting (3.5e Variant Rule)
From D&D Wiki
Semi-conservative spellcasting is a kind of compromise between the "conservative" (spell per day) system and the "liberal" (power point) system (used in psionics). It is designed to give spellcasters greater flexibility when they are in a pinch and should be balanced appropriately (e.g. limiting spell selection or reducing the number of spells per day). The nature of this system requires a class that can cast spells without preparation. Spellcasters who use this system have a spell per day list similar to those presented in the SRD. They can, however, exchange spells for their effective equivalent in other spells when they have difficulties. Spells can be exchanged "up" or "down" at a cost of one spell level i.e. the levels of the spell(s) to be exchanged are pooled into a total and redistributed according to the character's needs. Higher-level spells can be split into several lower-level spells, and several lower-level spells can be combined into a higher-level spell. In either case, one spell level is permanently lost from the character's spells per day as the energy expended in redistributing magic into different groups and organizations. 0-level spells count as 1/2 spell level, and extra spell levels accumulated because they cannot fit into an appropriate spell level are lost.
A sample semi-conservative spellcaster (8/7/7/7/7/6/6/6/6/6) has cast several spells earlier in the day, resulting in 5/0/6/7/7/5/6/6/5/6. He encounters a 1000-ft. pit and barely misses his save, falling in. He has several rounds before he will reach the bottom, so he splits a 9th-level spell into seven 1st-level spells (7 spell levels) and three 0-level spells (1.5 spell levels). The remaining 0.5 spell level is lost, and he casts feather fall and lands safely, taking no damage.