Low-key

Lighting that uses very little fill, so contrast between bright and dark is high and shadows hide part of the person. The "key" in the name refers not to the height of a light but to the brightness and contrast of the whole image. So it is not just a dark image: the defining trait is bright areas that stay crisp within the darkness.

Lighting positions seen from above. A single key light about 60° to the side of the camera, with almost no fill.

Camera view. Against a dark background only one side of the face stays bright, and most of the frame sinks into shadow.

How it feels

Darkness
Mystery
Tension
Strong contrast
Noir

When to use it

Film noir and hard-boiled crime films
Scenes where suspense builds
Scenes with a dark, mysterious mood
An interrogation room under a single bare bulb

In the movies

The Godfather (1972) · Source: Paramount Movies (YouTube) · Original video

Touch of Evil 1958
A noir by Orson Welles. Yale’s film analysis guide shows scenes from this film as examples of low-key lighting. The guide does not name a specific scene.

Not to be confused with

High-key: The exact opposite. It raises the fill to almost the same strength as the key, so there are almost no shadows.
Chiaroscuro: Chiaroscuro is an art term for strong contrast between light and dark; low-key is the name of the film lighting style that creates such contrast.
Underexposure: Just shooting the whole image dark can leave contrast low, which may not be low-key. Low-key means a big difference between bright and dark areas.

Prompt

Keyword

low-key lighting

Descriptive

Low-key lighting. A single hard key light with almost no fill, deep shadows hiding half the face, dark background.

Try these combinations

Close-up + low-key: an interrogation room
Close-up, low-key lighting, a suspect sweating under a single bare bulb in an interrogation room, deep shadows
Medium shot + noir style: an alley at night
Medium shot, film noir style with deep shadows and stark highlights, a detective waiting in a rainy alley at night