The grand drape, or main curtain on a stage, is typically much wider than the stage itself, because of the extra fabric needed to make the flowing pleats normally seen in such a curtain, and because the two halves of the curtain overlap. All that fabric needs substantial space backstage to keep the drape from being visible to the audience. For the curtain to be hidden behind the proscenium arch, two inches of backstage space is needed for every foot of curtain width (not stage width).
While most stage curtains consist of two drapes that part in the middle, some smaller theaters or home theaters use single curtains that pull open from one side. These curtains typically need a stack-back area equivalent to approximately 30 percent of the curtain width.
When designing a theater space or replacing a stage curtain, you will need to plan for stacking space in the wings. In addition, the curtain track ordered must be long enough to cover not just the stage but all the stack-back space as well.