The plaster line and the centre line are the two fixed reference lines a proscenium stage is measured from. One runs across the stage and one runs up and down it, and where they cross is the zero point of every scale drawing of that stage. A position on the set is given as a distance to one side of the centre line and a distance upstage or downstage of the plaster line, the way a grid reference fixes a point on a map. Between them, the two lines turn a drawing of a set into a set of measurements that people working in different rooms can build, tape out and strike from the same numbers.
They are worth understanding as a pair, because they do one job together. Either line on its own only measures in one direction. The centre line tells you how far left or right something sits but nothing about its depth; the plaster line tells you how far up or downstage but nothing about its side. Take a reading from both and the point is fixed exactly. Almost every other drawing in a production, the section, the lighting plot, the setting list, is either laid over these two lines or figured from them, so most theatre paperwork quietly assumes you can already find them.
The centre line
The centre line runs the depth of the stage, straight down the middle from the front edge to the back wall, dividing the stage into a stage-left half and a stage-right half. (Stage left and right are named from the actor's point of view, facing the audience, so on a plan drawn the usual way round they fall on the opposite sides of the page.) It is the more intuitive of the two, because it is genuinely central: it bisects the proscenium opening, and in a well-built house it lines up with the middle of the auditorium, so a set placed symmetrically about it reads as symmetrical from the best seats.
On a drawing the centre line is conventionally drawn as a chain line, a long-dash short-dash pattern used for nothing else, and labeled CL, sometimes with the surveyor's centre line symbol. Every sideways measurement on the plan is given as so many meters left or right of it. Because it corresponds to something real and permanent in the building, it is easy to find on the actual stage: measure in from each side of the proscenium, split the difference, and the point can be marked and used for the life of the theatre.
The plaster line
The plaster line runs the other way, straight across the stage from side to side, at the upstage face of the proscenium arch. Every depth measurement, how far up or downstage something sits, is taken from it. The name is a piece of history: in a traditionally built theatre the proscenium was a plastered masonry arch, and the plaster line is where the face of that plaster met the stage floor. Plenty of modern prosceniums are not plaster at all, and the name has outlived the material, but the line it marks is the same, the boundary at the front of the stage where the architecture of the house stops and the stage proper begins.
The plaster line is the harder of the two to pin down, for two reasons. The first is that a proscenium has thickness, so "at the arch" can mean the downstage face, the upstage face, or somewhere in the reveal, and different houses and drawings settle it differently; the upstage face is the common choice, but it is worth confirming rather than assuming. The second is that the plaster line is not always the line a plan is actually dimensioned from, which is where the real confusion starts.
Reading a position from the two lines
Where the two lines cross is the origin, the zero point the whole drawing is measured from. Give a position as its distance from the centre line and its distance from the plaster line and you have fixed it completely: a chair set 1.2m stage right and 3m upstage sits at one point on the stage and nowhere else. This is why the two lines matter more than anything else on a groundplan. They are what let two people who have never met build the same set in two different workshops and have the furniture land in the same place. For how the rest of a plan is read around them, see how to read a theatre groundplan.
It is worth asking why the walls of the building are not used instead, since they are already there and already fixed. Three things rule them out, and a touring production meets all three in its first week. Walls wander: real buildings, particularly old ones, are out of true by amounts that never show to the eye but matter at stage scale, and a measurement taken from a wandering surface wanders with it, where the two lines are geometric constructions rather than masonry and stay exactly straight and exactly square whatever the room is doing. Walls are also not where the design lives, because a set is placed relative to what the audience sees through the arch rather than relative to the scene dock behind it. And walls do not travel, while the two lines do: a design figured from them can be laid down in another house by drawing the same two lines there and repeating the same numbers.
The measurements do not stay on paper. Early in rehearsals the stage management team tape the plan out at full size on the rehearsal-room floor, and the first two things down are usually the centre line and a line standing in for the front of the set, with the marks for walls, furniture and entrances measured out from them. The same lines are snapped onto the stage itself once the set is built, so that the carpenter, the stage manager marking the floor and the crew resetting a piece in a blackout are all working to the same origin. A spike mark on the deck means something only because it was measured from the two lines, and it can be found again the same way if it is lost. Directions are usually written out in words, "2.5m SL" or "3m US of plaster line", rather than given as plus or minus signs.
Putting the lines down is the first job of most fit-ups, and it is done in a fixed order. Centre comes first: measure the proscenium opening, mark its midpoint at the plaster line and again a few meters upstage, then snap a chalk line through both marks. It is worth checking that result rather than trusting it, because the architectural centre of an old proscenium is not always where symmetry says it should be. The plaster line is then run across the stage along the upstage face of the arch, square to the centre line, with a large builder's square or the 3-4-5 triangle method keeping the right angle honest, and where the two cross is marked as the origin. After that the plan sets itself out. Each position on the sheet is two numbers, so each spike mark on the deck is two tape measurements, one along from the centre line and one up from the plaster line, and two people with two tapes can mark out a full set in well under an hour. The marks agree with the drawing because they were made the same way the drawing was.
The setting line, and the lines it is confused with
The plaster line is a fixed feature of the building. The setting line is a decision. It is a line the designer chooses for a particular production, usually at or just downstage of the front of the set, and figures the scenery from that instead of from the plaster line. On a stage with a deep forestage or apron, the set often sits well downstage of the arch, and it is easier to measure from the front of the set than from a plaster line several meters behind it. When a plan is dimensioned from a setting line, its depths are relative to that line, not to the plaster line, even though the plaster line may still be drawn on the sheet.
The plaster line and the setting line are not the same line, and a set built to the wrong one lands the whole design at the wrong depth on the stage. Check which line a plan's depth measurements are figured from before working to them, because both may appear on the same drawing.
A couple of other lines get named in the same breath and are worth telling apart. The iron or safety-curtain line marks where the fire curtain drops, close behind the plaster line but not the same as it, and the smoke pocket the iron runs in is sometimes used as a convenient physical reference on stage. None of these is the datum a plan is measured from unless the plan says so. The habit that saves trouble is the same in every case: find the line the sheet actually uses, from its key or its title block, rather than assuming which line the numbers count from.
Stages without a proscenium
The plaster line belongs to the proscenium. On a thrust stage, in the round, on a traverse or in a bare studio, there is no arch to measure from, and often no single front to the stage at all. What happens then is that the company agrees a datum in its place: a setting line ruled across the plan, or a chosen reference point, that everything is measured from for that production. The centre line usually survives in some form, though in the round it may become a center point with more than one axis through it, since there is no single audience side to face.
The principle holds even where the plaster line itself does not exist: a stage is worked from a fixed origin and two directions, whatever the shape of the room. What changes from one configuration to another is which line plays the part, not the need for one. What each shape asks of a design is its own subject; see stage configurations.
The two lines in Setplot
Setplot builds the plan on these two lines rather than on a blank canvas. The centre line and the plaster line cross at the origin, and every piece of scenery is held as a real measurement off them, in meters, so a piece's position is a distance from the two lines rather than a shape pushed roughly into place. Across the stage, stage left counts as positive and stage right as negative; in depth, upstage of the plaster line is positive, so a position on the apron in front of the arch reads as a downstage, negative depth. Because the plan is metric and to scale throughout, a measurement taken off a printed sheet is the measurement on the stage.
Two useful things follow from measuring everything this way. A design can hold several venues, and switching from one to another leaves every mark where it is and changes only the room around it, because a mark is stored as its distance from the two lines and not as a spot in one particular building, which is what lets a set travel to a second house on tour. And the same origin carries into the paperwork the plan feeds: the lighting plot hangs its bars at trims measured off the plaster line, the section puts back the heights a plan leaves out, and every export is figured from the same zero. The core tools are free, including the public drawing board before you make an account, which is a reasonable place to see how the two lines organize everything else on the plan.
For all that they are the faintest marks on the sheet, a broken line down the middle and a line across the front, the plaster line and the centre line are what a groundplan is built on. They give every point on the stage an address, and that is what lets a set drawn once be built, taped out and lit by people who never once stand in the same room.