Some older editions of D&D require a caster to have specific spell components on hand to be able to cast a spell. This is both a method of limiting the power of magic and providing a slightly more "magical" feel to the game. Unfortunately, it is such a pain in the ass to track that few groups regularly use it. Here is a method of using spell components which requires less far bookkeeping.
Casting an arcane spell requires various components which are consumed in the spell. Most spells only require the generic components discussed here. However, the GM may require very specific components to cast specific rare or powerful spells -- such components will have to be bought or found. Conversely, the GM may decide that a few spells (like "power word" spells) do not require components to cast.
Generic spell components can be obtained in most towns and cities. One simply purchases a number of gold pieces worth of generic components. Each 500 gp worth of generic components weighes one pound. When an arcane caster wants to cast a spell, he or she pulls the appropriate amount of generic components out and casts the spell. Spells require their spell level squared time ten gp worth of spell components. That is, a first level spell requires 10gp [(1*1)*10gp] worth of spell components to cast, a second level spell requires 40gp [(2*2)*10gp] worth of spell components to cast, a third level spell requires 90gp [(3*3)*10gp] worth of spell components to cast, a fourth level spell requires 160gp [(4*4)*10gp) worth of spell components to cast, etc.
A rule like this one provides most of the benefits of requiring spell components to cast spells, but eliminates much of the boring list keeping. The GM still has the optional to require specific special components for spells that he considers very powerful or very hard to use in his campaign. It's up to the GM as to what happens if a character doesn't have the needed spell components (he ran out, they were stolen or lost, etc.). It might be impossible to cast spells without components or spells might cost more in other areas to cast without components -- perhaps taking many minutes to cast or costing more spell points (in systems with spell points.
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