A Thaum Is The Basic Unit Of Magical Strength. It Has Been Universally Established As The Amount Of Magic Needed To Create One Small White Pigeon Or Three Normal Sized Billiard Balls.

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A Thaum is the basic unit of magical strength. It has been universally
established as the amount of magic needed to create one small white pigeon
or three normal sized billiard balls.
-- Terry Pratchett, The Light Fantastic

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