I don't like the term 180degree line because it simply isn't there. It's not a line. It's not a geometric shape... For this reason I call it an eyeline. It isn't determined by camera placement, or by actor blocking. It is determined by where the characters are looking. If you are standing there having a conversation with 2 other people, when they are talking, you are on one side of them. You remain on this side if you or they stand still. So it makes sense that Person A (to your left) talking to person B (to your right) is looking at person B, then they are facing to the right of you... camera right. Likewise person B is looking at person A so they are facing to the left of you. This is what makes that cut ok. It's as if you stepped forward to see their face a bit more and are turning your head back and forth through out the conversation. What happens if Person B looks off to your right as a friend shows up from behind you? Well naturally you turn around and look off to the side (logically your right side) and see your friend approaching. This has just established a new eyeline because person B has crossed the camera with his eyes. The same is true if Person B walks to your left side (effectively crossing the 180/eyeline) or if you walk around, or between them moving to the other side. (the camera crossing the 180/eyeline) There are many great films that use this for more dynamic shooting. Crossing the 180/eyeline is an amazing tool as long as you know how to manipulate it so you aren't confusing the audience... Unless you want to. :) Anyway, I like to think of it as an eyeline because I think of the camera as an active character in the film... In fact it is... it is the audiences character!