Plaster Line

Plaster Line:
A reference point onstage that runs perpendicular to the center line of the stage, usually denoted by the upstage side of the Proscenium Arch. Literally where the Plaster of the wall lies. Usually the plaster line is the upstage start of the architectural elements of the theatre that the audience always sees. In some theatres, such as a black box, the plaster line is the Back wall of the theatre, as there is no Proscenium Arch. Never the less, this is the plane from which all upstage - downstage measurements are taken to determine placement of scenic elements.
The plaster line is usually denoted by a "long-dash" line in a lighter line weight than the center line. (I've also seen it as long dash, two dots, long dash, repeat as nec.)
Can also be called the fire line, due to the fact that the fire curtain and the smoke pockets that the fire curtain drops in are just upstage of the plaster line. Some theatres will not allow any scenic elements to pass beyond the plaster line due to the risk of the fire curtain being caught up on those scenic elements. And to further add to the confusion, sometimes all dimensions are taken from the "smoke pocket line"--the upstage side of the smoke pocket steel which is 3"-6" upstage of the plaster line. This becomes particularly important when laying an all-over stage deck.