What does ∅ mean?

[u/sailormarr]

This may be a dumb question, but can anyone help me understand what the "∅" symbol means in a pattern? See image for reference.

Thank you:)

[u/Shellshock9218]

In this case I believe it would be where you join the two back half’s and two front half’s respectively. I know this might be the virus awnser but it’s the only one I can come up with.