Slugline

A slugline is the capitalised scene heading in a screenplay that states whether a scene is interior or exterior, where it takes place and at what time of day.

The standard form is three parts on one line:

INT. DINER — NIGHT

INT. or EXT. (interior or exterior), the location, then the time. It is set in capitals, it sits on its own line, and it is what tells a reader — and later a first assistant director — that a new scene has begun. Sluglines are how a script is broken down for scheduling, which is why the convention is rigid: a location spelled two ways becomes two locations on the board.

A secondary slugline, sometimes called a mini-slug, marks a shift within the same scene — KITCHEN, LATER, BACK TO SCENE — without the overhead of a full heading.

The use worth being strict about is time travel. Open a flashback by marking it in the heading, and close it explicitly, every single time: END FLASHBACK, or RETURN TO PRESENT in the next slugline. A reader who cannot tell which decade they are in has stopped reading the script and started reconstructing it, and the flashback has cost more than it delivered.

Prose has no equivalent convention, which is why novels do the same work with a scene break, a date line, or the first sentence.