Requisite
Any Hermetic spell is based on two Arts, a Technique and a Form. However, some spells require additional Arts to control or direct the magic. A paradigmatic example would the Curse of Circe spell, that changes a human into a pig: this is a Muto Corpus effect, but also requires Animal to shape the new form of the body.
Such extra Arts are called "requisites". When casting the spell, or inventing it, choose the lowest of any Technique requisite and the spell's Technique, and the lowest of any Form requisite and the spell's Form.
Adding requisites to a spell may increase the spell's level when these requisites merely refine a basic effect. For example, a spell that creates fire that doesn't burn the magus may be a Creo Ignem spell with a Rego requisite. Since the main effect (creating fire) can be created without the secondary (Rego/control) refinement, such refinement should require a higher spell level.
Note that Hermetic spells can still only do one thing. You should avoid the tendency to pile on requisites to represent compound spell effect. For example, one could imagine a spell that both wounds the targets (a Perdo Corpus effect) and burns them (a Creo Ignem effect), but it would probably be wiser to not allow such a combined effect in a single spell.
It is also possible to add requisites on the fly to some spells and effects. For example, one can use the Unseen Arm spell to move objects in general, using casting requisites to target things made of animal products, plants, or so on. Such requisites lower the Arts in casting as normal. Adding casting requisites isn't usually possible for most effects, but may be if the spell guidelines say so or the storyguide does.
References
- See the core rulebook, page 114.