Key light

The main light on a face, the one that decides the mood. Move it a few degrees and a character turns from open to hidden.

The key is the scene's sun: the dominant source that models the face and sets the moral weather. Its position is character work, frontal and soft reads open and honest; high and side-on carves doubt into the sockets; from below, the face becomes a warning.

A few degrees is not an exaggeration: the difference between trust and menace on the same face is a light stand nudged. Everything else, fill, backlight, negative fill, is negotiation with the key's decision. In our visual language the key is almost always single and warm, one gold source against a cold world, which is the studio's face in lighting form.