The Meaning of the Open Pentagram

[Mephistopheles] ... Might I, perhaps, depart at present?

[Faust] Why thou shouldst ask, I don't perceive. ...

[Mephistopheles] ... forth I may not wander. My steps by one slight obstacle controlled,--The wizard's foot...

[Faust] The pentagram prohibits thee? ...

[Mephistopheles] Inspect the thing: the drawings not completed. The outer angle you may see, Is open left--the lines don't fit it. 

Goethe, von, Johann Wolfgang,  Faust: A Tragedy, trans. Bayard Taylor, Leipzig: F. A. Brockhaus, 1872, pp. 49, 50.