Near obsolete piece of stage hardware, used to hold flats in a vertical orientation.
From http://www.northern.edu/wild/th241/sc96.htm:
A stage brace, a standard piece of theatrical hardware, is two overlapping pieces of hardwood which can be adjusted in length. The top of the brace contains a "double pronged curved hook" that fits into the eye of a brace cleat. The bottom of the brace is attached to the stage floor with a stage screw. The cost of a stagebrack [sic] is about $60.00.
In modern usage, has almost completely been replaced by the stage jack.
From http://www.northern.edu/wild/th241/sc96.htm:
A stage brace, a standard piece of theatrical hardware, is two overlapping pieces of hardwood which can be adjusted in length. The top of the brace contains a "double pronged curved hook" that fits into the eye of a brace cleat. The bottom of the brace is attached to the stage floor with a stage screw. The cost of a stagebrack [sic] is about $60.00.
In modern usage, has almost completely been replaced by the stage jack.