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 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.