The capitalized line that opens every scene: INT. or EXT., the location, and the time of day.
A scene heading - INT. COFFEE SHOP - DAY - tells the reader and the crew three things: are we inside or outside (INT./EXT.), where are we, and roughly when. Every new scene gets one, and production departments schedule and budget straight off them.
Keep locations consistent (INT. JANE'S APARTMENT, not three variations of it) - in production, each distinct slug is a set.
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