Most people mistake Summoning Grids to look like pentacles.
First appearence in The Atrocity Archives training course with Fred from Accounting.
Uses of a Summoning GridEdit
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Andy creates a summoning grid in his office for practice using off-the-shelf components, including an Arduino in which to summon a Class 1 voice-responsive agent [a demon] and make it do a handstand, then send it away again. (The Rhesus Chart pg 10)