GURPS - Magic - Arts and Techniques

Most magic users are practitioners of the "Arts and Techniques", a broad magical conceptualization that represents a quasi-scientific view of magic as an impersonal energy field that binds all reality together. In GURPS, it is called "mana", but it could be known by many other names just as well - qi, vis, animus, the Force, et cetera. This energy responds to the focused will of the practitioner, allowing for changes outside of natural law. The term "Arts and techniques" actually refers to a wide array of magical schools of thought, all of which share the general concept of magic as energy. In application, the Arts and techniques produces magic that is fast and flexible, but relatively limited in scope and scale. "Arts and Techniques" spells use all of the standard GURPS Magic rules, with several important modifications.The core skill of the "Arts and Techniques" is called "Thaumatology". No spell may be learned to a level higher than the caster's skill level of Thaumatology.

In the event of a Critical Failure, the caster may roll against Thaumatology at a penalty equal to their "Margin of Failure" on the critically failed spell. If the Thaumatology skill roll is successful, the critical failure is converted into a simple failure - a close call, but disaster is avoided. 

Thaumatology skill allows for the use of "Cantrips", trivial spell effects specific to each college known to a mage. With a 1 second of concentration and a Thaumatology roll at no penalty, a magic user can generate a very minor magical effect appropriate to the weakest possible use of the minor spells of a college.... lighting a candle with Fire College, using Light and Darkness college to produce a dim glow in the hand suitable for reading a page, snatching a key off a shelf with the Movement College, summoning a single annoying fly with Animal College, and so forth. 

 Spells can be adjusted as per the rules in GURPS Thaumatology, p39, trading fatigue and a skill penalty for various improvements, such as increased range, area of effect, potency, and so forth. In general, each +1 energy and -1 skill penalty allows for a +5% enhancement to a spell. Each such adjusted spell counts as a separate Average Difficulty technique if the mage desires to learn it as a spell in their repertoire. While most practitioners of the Arts and Techniques master well-known versions of their spells, there is a lot more potential variety available to skilled casters.

Theoretically any spell can be cast at default or at least attempted, so long as the caster has at least one spell in the same college. It is difficult, fatiguing, and can be very dangerous. The default penalty is harsh; the caster's effective skill level is equal to the base spell  - (Prequisite Count + 4). Energy Cost and Time to Cast are both doubled, and any failure is a Critical Failure! For example, a mage has Create Fire-16. She wants to cast "Explosive Fireball"... prerequisite count of 4 means a -8 skill penalty, so she can attempt it at a skill of 8. It is usually safer to learn the spell the old-fashioned way, but sometimes in a pinch one must improvise.

Skill in the Arts and Techniques allows a caster to access “Threshold Magic” in dire emergencies… (GURPS Thaumatology p76). This allows the casting of some very powerful magic indeed, but at a very high cost to the caster and possibly the area around him. The basic Threshold is 0, meaning any casting calls for a Calamity Roll. Recovery rate is equal to Magery + 1. Additionally, the caster is allowed to make a Thaumatology + Magery skill roll – subtract the Margin of Success or Failure from the Calamity Roll, giving a chance to reduce the severity of the inevitable backlash... or to make things far worse. Threshold magic should not be attempted lightly, and is in fact banned in many regions.