- "I'm trying to find a pattern. Why some surges and not others."
"Well, if you'd had a proper Brakebills education, you'd know the answer. Circumstances, dear. The phase of the moon, the nearest body of water. You must factor them all into your casting."
"Even us filthy hedges know about Circumstances."
- ―Julia Wicker and Henry Fogg
Circumstances are forces that affect a Magician's spell during casting. There are four tiers of Circumstances that Magicians must account for: Major, Minor, Tertiary, and Quaternary, and each tier has several exceptions, irregularities, and special cases that Magicians must account for to successfully cast a spell.
The same way a verb has to agree with its subject, even the simplest spell had to be modified and tweaked and inflected to agree with the intention, purpose, and exact conditions of its casting. For most spells, Circumstances that Magicians have to take into account include those concerning the time of day, which direction they're facing, and the ambient temperature. For more complex spells, Circumstances to be accounted for include phase of the moon, the dew point, and internal Circumstances such as the balance of bodily humors; melancholic people, for instance, cannot perform transfiguration spells.
Furthermore, a spell's Circumstances can be adjusted by Magicians in order to perform irregularly, in some cases augmenting the spell from its base form. Alice Quinn purposefully altered the Quaternary Circumstances for the Miller Flare to release heat during the race to the south pole in Antarctica, and later adjusted her casting of the Magic Missile during the Battle at Ember's Tomb against The Beast, releasing an entire flock of light daggers.
After memorizing the four tiers of Circumstances, Magicians must then memorize the hundreds of Corollaries and Exceptions that must be accounted for in any given situation.
- The five Tertiary Circumstances are Altitude, Age, Position of the Pleiades, Phase of the Moon, and Nearest Body of Water.
- The mnemonic "Please Take My Drunk Aunt Home" can be used to remember minor inflections Phrygian, Thracian, Minoan, Doric, Alexandrian, and Hittite.
- During a full moon, the ring finger takes the Pythian mode for casting.