This one’s a bit more complicated to explain. Basically though, if say you are filming a conversation between two people – let’s call them Jethro and Tull – picture an imaginary line running between them (and extending out of the back of their heads to infinity).

We call this the 180° line (or the axis of attention, or a number of other terms which all refer to the same thing). Any and all shots of the two-way conversation need to be filmed with the camera on one or t’other side of this line.

Otherwise – if the camera crosses the line – Jethro and Tull will constantly appear to the audience to be switching places. Jethro will be on the left side of the screen and Tull on the right, then suddenly Tull will be on the left and Jethro on the right, then Jethro will be back on the left again and so on. And by now, your audience has a headache. Just as with incorrect screen direction, even though most of them won’t know what is actually wrong, they will know that something most definitely is.^[1] And it’s likely either they’ll come away thinking that it was the actor, or the writing, or the lighting – or they’ll just change channel. So you can think of line crosses as just like cross lions – they are something very much to be avoided.^[2]

Sounds pretty simple right? Well, not as much as you might think. For starters, most shots are cheated, even if only a little – actors and set are moved around either to produce a better frame, or simply because it’s not possible to shoot otherwise (e.g. there is a large wall where you would like your camera to be. Or a big expanse of nothing off the edge of a cliff).

For that matter, if we’re shooting that conversation between Jethro and Tull, then there’s a fair chance that we’re shooting Tull’s close-ups at a different time and maybe even on a different set. So someone has to have a record of how we shot Jethro’s half of the conversation.^[3][4]

Oh yes, and of course the aforementioned imaginary line moves. A lot. So it needs to pivot – or rather, it needs a pivot, an arbiter of attention, someone (or something) on screen to change it. Which needs to be filmed. And so on.

And… keeping track of all this and preventing line crosses is the Script Supervisor’s job.^[5].

One more thing. Don’t listen to anyone who says there is no line with only one character on screen. They’re wrong, there is, there always is, just trust me on this one. And if you don’t, give me a call and I’ll happily explain it in more detail.

1. Of course, if it’s say a horror film then this may be exactly what you want – that feeling that there is something wrong, but you don’t know what. But then you’ll have planned this, so will still be paying very close attention to your line crosses. [↩︎]
2. Sorry – that works better when answering this question in person… [↩︎]
3. I think it’s the bit in Lord Of The Rings: The Return Of The King with Sam and Frodo talking in Shelob’s lair for which they shot Sam’s half over a year after Frodo’s. [↩︎]
4. Funnily enough, this hasn’t come up so much in my career to date. [↩︎]
5. He’s my lion. Down, boy. [↩︎]