Screen Grammar

These are the rules that keep an audience oriented when shots are edited together. Break them deliberately and you get unease; break them by accident and you get confusion.

The 180-degree rule

An imaginary line called the axis of action connects the characters. The camera stays on one side of it, so screen direction stays consistent. If A is on the left and B is on the right, they stay that way in every shot, and when you cut between their close-ups they appear to be looking at each other.

Two characters joined by the axis of action, with three permitted camera positions above the line and one forbidden position below it axis of action A B Any camera on this side is safe Crossing the line flips screen direction they face each other both look the same way
The two frames at the bottom are what the audience actually sees. Cross the line and B suddenly appears to be looking away from A, and the geography of the scene silently collapses.
Break

How to cross the line on purpose. Move the camera visibly across it in a single shot so the audience travels with you; cut away to a neutral shot first (a clock, an object, a head-on frame) to reset the line; or simply break it for effect, as Kubrick does in the bathroom scene of The Shining to make the space feel psychologically wrong.

The 30-degree rule

The companion rule: between successive shots of the same subject, the camera needs to move at least 30 degrees. Cutting from one angle to an almost identical one reads as a jump cut: a mistake, not a choice.

Too similarThe cut reads as a glitch in the image, not a new view.
Genuinely new angleThe audience accepts it as a different view of the same thing.

The rest of the grammar

Shot / reverse shot
The standard dialogue pattern: A's shot, then B's matching shot, back and forth. "Matching" means similar size, height and lens; mismatched reverses make one character feel weirdly more important.
Eyeline match
If a character looks off-screen, the next shot shows what they are looking at, from a matching angle. If they look down, we should be looking slightly down at the object too.
Match on action
Cut during a movement (someone sitting down, opening a door, turning their head) so the action continues across the edit and the cut becomes invisible.
Screen direction
If a character exits frame left, they should enter the next shot from frame right to read as continuous travel. Reverse it and they appear to have turned around.
Rule

Grammar is a budget, not a cage. Every rule here exists so the audience spends no attention working out where they are. Spend that attention only when the confusion is the point.

Watch

The 180 Degree Rule in Film (and How to Break The Line) (StudioBinder), on holding the line, crossing it, and bending it on purpose.