Draw a line between two people who face each other. Keep every camera on one side of it.

Use it when. If you cross it, the two people swap sides of the screen and seem to look the same way. Cross it only with a move that shows the crossing.

Between two shots of the same subject, move the camera at least 30 degrees, or change the size by two steps.

Use it when. A smaller change looks like a mistake: a jump cut. WS to MS, or MCU to CU from a new angle, cuts clean.

If A looks frame-left in her single, B looks frame-right in his. Their eyes should meet across the cut.

Use it when. Set the eyeline just beside the lens, on the side of the other actor. Note the side on the shot list.

Cut in the middle of a movement: the hand reaching, the head turning, the door opening.

Use it when. The movement hides the cut. So shoot the whole action in both sizes, with an overlap, never half in each.