Masking is the hiding of the paraphernalia of the theatre from the vulgar gaze of the audience. The term also applies to the equipment used to this end. Fundamentally, there are two requirements: to hide the wings and to hide the flies. This is done with side masking and top masking respectively.

Top masking is achieved by the use of borders and occasionally headers. If the first border is immediately behind the proscenium arch it is called the house border or teaser and may form part of a false proscenium . Usually, top masking only has to hide the on-stage electrics , but sometimes the bottom of flown scenery as well.

Side masking is achieved using (black) maskers and legs. The main problem is hiding the cast and stage crew in the wings rather than equipment. It should be possible to walk the length of either wing, and reach all entrances, out of sight of all of the audience. If this is the case, the wing is said to mask. The limits of the audience's view are sight lines and are sometimes marked on the floor with tape; such marked lines are called idiot lines.