In theater, a brace is a sliding piece of wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

 or metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 with a 'butterfly' winged nut to make it longer or shorter to fit the flat used to stabilize a flat set piece such as a flat

Flats (theatre)

Flats, short for Scenery Flats, are flat pieces of theatrical scenery which are painted and positioned on stage so as to give the appearance of buildings or other background. They are also called backdrops or backcloths....

. The nut is used ,so that it can be changed more quickley than a screw to the floor during a quick change. Usually, a brace is painted black to make it less noticeable to the audience. Braces are often used to form a triangle between two perpendicular items (like a vertical flat and a stage

Stage (theatre)

In theatre or performance arts, the stage is a designated space for the performance productions. The stage serves as a space for actors or performers and a focal point for the members of the audience...

. They can also make a flat piece stronger by forming an X-shape between all four corners. Another way to brace a rectangular flat is to use special braces, called toggles which run at regular intervals, parallel

Parallel (geometry)

Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...

to the short end of the flat, effectively breaking it into many smaller, stronger rectangles.