Invisible line between actors or actor and object — governs where the eye travels. Cross it and the audience feels spatially confused.
The sight line determines the visual flow of a scene. When two actors look at each other, an imaginary line is created between their gazes – the axis. Everything you film afterward must respect this line, otherwise the viewer's perception will jump around wildly. It's not about rules for the sake of rules, but about spatial clarity.
In practice, it works like this: You film actor A from the left; he looks to the right of the frame. Then you switch to the opposite side – actor B is now on the right of the frame and looks back to the left. The axis runs horizontally between them. As long as you don't cross this line, the viewer immediately understands the spatial relationship. If you cross it – meaning you suddenly film from the other side of the axis – the entire space flips around. The two actors now seem to be looking in the same direction. This is disorienting and only works if it's intentionally desired (e.g., for psychological confusion or deliberate spatial jumps).
The axis also applies between an actor and an object. A character looks at a table – the axis runs between their eyes and the table. If you change the camera position, you must stay on the same side of this line. This becomes particularly important in dialogues: when switching between close-ups, ensure that both directions of gaze remain consistent. A common mistake is to unconsciously cross the axis and then realize in the edit that the heads are pointing in the wrong direction.
Exceptions are allowed – for example, by using a neutral camera positioned exactly on the axis (frontally on both actors), which establishes a new perspective, or by a deliberate axis jump as a dramatic stylistic device. Some directors deliberately play with axis breaks for tension or unease. But this only works if the audience has internalized the original axis. Those who know the rule can break it.