Single

One person, alone in the frame, with nobody else in it. That isolation is the content: the frame agrees with a character who feels alone, or disagrees with one who is not.

It is also the workhorse of dialogue coverage. Most conversations are cut from two singles, and the audience assembles the room from them without noticing.

Any size can be a single, from a wide to an extreme close-up. What defines it is that only one person is in the picture.

A clean single has no part of anyone else in it. A dirty single lets a shoulder, an arm or a blurred head sit at the edge of frame, which keeps a trace of the other person in the room. The two say different things and both are useful.

Use it when a character has to be alone with a thought, and use it in pairs when two people are talking and you want to control what each line costs.

Move off it when the relationship is the point. A two-shot shows the distance between people, and singles hide it.

Over-the-shoulder coverage keeps a soft shoulder in the near corner, which makes it a dirty frame by definition. A clean single removes that shoulder and with it the sense of someone listening.

A two-shot puts both people in one frame. If the audience can see both faces at once, it is not a single.

Models add a second figure to fill the space, especially in wider frames. Say the subject is alone and nobody else is in frame.

The other problem is a frame that looks composed for two. If the subject sits hard to one side with a large empty area beside them, the shot reads as somebody missing. Either centre the subject or make the empty side deliberate.