Spells are built using Arts, which are divided into Techniques and Forms. A Technique is like a verb, and describes some action or process. A Form is like a noun, and describes the subject or target of the action. Most spells use one art and one form, though some complex spells may require three or more arts. A wizard purchases arts as specific skills.
Each spell is constructed using the Gramarye system, which is used to determine difficulty and cost.
Spells are cast using the lowest required skill. For example, a create fire would use the lower of Create or Fire. Each spell has an associated difficulty, which represents power and complexity. A successful casting requires a skill roll which meets or exceeds the difficulty of the spell. A failure means the spell did not work; a fumble (a natural or figured result of -4) means the spell worked, but did something other than what you wanted!
The universe resists being manipulated magically. Too much magic causes dangerous side-effects; these usually affect the wizard, but in extreme cases can cause changes to the entire world! To represent this, each wizard has a Threshold number which indicates how much magic he can work before the universe begins to unravel. The wizard also has a Tally indicating his current total. Think of the tally like a temperature guage, and the threshold like the start of the "red zone." Each spell has a point cost; when a spell is cast successfully, this cost is added to the current tally. Whenever the tally ever exceeds the threshold, the wizard must make an Calamity Check. (Note that failed spells have no cost since nothing happened, but botched spells are at full cost.)
Most wizards have a threshold of 10 + Intelligence (this can be adjusted with Virtues and Flaws). The magical tally recovers 1 point every hour.
A wizard can enchant permanent magical items. The base cost for such an enchantment is three times the normal cost for the spell. For enchantments, the cost must be paid in Vis, and requires a minimum time of 1 day per point. In rare instances, wizards can create items almost instantly. To do this, the cost is five times the normal cost, payable in Vis, and it must be cast as a ritual spell (with no bonus). When performing an enchantment, effective skill is limited by the Enchant skill.
Scrolls are simply spells embedded into a parchment. Writing a scroll is a standard ritual casting, and is paid in Vis instead of tally points. A scroll can be used by anyone who can read the language in which it is written, though it is common to design scrolls which require special knowledge or procedures to activate. When writing a scroll, effective skill is limited by the Scribe skill used.
Potions are created by alchemical processes using the Alchemy skill. Once the effect of a potion is determined, a level is assigned. It can then be produced just like a scroll, with the same time and vis requirements as the equivalent Ritual spell.