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In the magical rituals of Wicca, a magic circle is cast before the main magic is performed.
The magic circle serves to create a magical cylindrical barrier that prevents the main magical power
that is created within the circle from dispersing, so that the magical power can build up.

The magic circle may be cast with either a sword or an athame, which is a black-handled knife.

The practice of casting a magic circle, and doing so with a sword or black-handled knife,
was taken from chapter 9 of book 2 of the Key of Solomon,
though in the Key of Solomon the circle serves a different purpose.