Sound and action occur within the visible frame — actors speak, objects make noise. Opposite of off.
In editing, the dramaturgy of sound and image is decided at a fundamental boundary: On means we hear what we see. The actor moves their lips, we hear their voice. The car drives through the frame, its engine sounds from the same direction. This congruence of visual and acoustic information creates authenticity—or at least the illusion of it. On set, this is trivial; in editing, it becomes a strategic decision.
In practice, we distinguish between true On—sound was recorded in sync with the image—and synthesized On, which the sound designer adds later. An actor speaks their lines on location, the recording is perfectly in sync: that's true on-sound. Later, we realize the quality is poor or we prefer a different take—now we dub a different voice over it and call it ADR (Automated Dialogue Replacement). The new voice is precisely synchronized with the actor's mouth movements but remains artificial. The viewer still sees on—the lips move in sync with the words—but doesn't notice that we discarded the original.
The opposite, Off, gives us enormous dramaturgical freedom. A voice-over creates commentary, inner monologue, or distance. Street noise is heard, even though the car is not in the frame. This divergence of the visible and the heard allows for layers, subtext, tension—it is the craft of modern editing. On, on the other hand, binds us to the immediate present, to what is there. It creates presence.
The most common challenge: A scene was shot on set with weak audio. In editing, we see clear lip movements, but the recorded dialogue is barely understandable or full of noise. Now we have to re-record the voice—and must not deviate from the sync, otherwise the entire scene will feel wrong, the suspension of disbelief will break. This is the price of the on-convention: perfection is visible, every mistake too. With off-sound, we would have more leeway.