A flat (short for scenery flat) or coulisse is a flat piece of theatrical scenery which is painted and positioned on stage so as to give the appearance of buildings or other background.

Flats can be soft covered (covered with cloth such as muslin) or hard covered (covered with decorative plywood such as luan). Soft-covered flats have changed little from their origin in the Italian Renaissance. Flats with a frame that places the width of the lumber parallel to the face are called "Broadway" or "stage" flats. Hard-covered flats with a frame that is perpendicular to the paint surface are referred to as "Hollywood" or "studio" flats.

Usually flats are built in standard sizes of 8 feet (2.4 m), 10 feet (3.0 m), or 12 feet (3.7 m) tall so that walls or other scenery may easily be constructed, and so that flats may be stored and reused for subsequent productions.

Often affixed to battens flown in from the fly tower or loft for the scenes in which they are used, they may also be stored at the sides of the stage, called wings, and braced to the floor when in use for an entire performance.