Sightlines fail quietly. A design can be beautiful on the model, correct at the workshop and completely wrong for forty seats on one side of the house, and nobody finds out until the first preview, by which time the fix costs a build day instead of a pencil line.
The check itself is not hard. You take the seat at the end of the front row, draw a line from it past the edge of the proscenium, and continue it upstage. What that line tells you is two separate things, and this tool draws both of them for a stage you describe in four numbers.
The two things one line tells you
Everything on the far side of the line is invisible to that seat. That is the upstage corner on the seat's own side, and it is where an actor gets lost. The tool shades the area that both ends of the row hold and hatches the corners they lose, so you can see immediately whether the upstage corner is somewhere you can stage anything.
Everything on the near side of the line, offstage, is visible to that seat. That is the crew, the ladders, the back of the scenery and the wall of the building, and it is the reason masking exists. The same construction from the opposite seat, past the opposite edge of the opening, tells you how far into the wing that seat can see at any depth, and the tool prints it as a short table.
Read that table as an exposure, not as a leg position. It says how much wing is in view at each depth, which is what your masking has to deal with. It does not say where to hang the leg, because where a leg goes depends on what the legs downstage of it already hide, and that is a chain rather than a single line. What the table is good for is the shape of the problem: the figure grows quickly with depth, and once it is wider than your wing space the whole wing is in view at that depth and the masking has to come from something further downstage.
Those two lines are why the drawing has two colours. One is about what the audience misses. The other is about what they should not have caught.
Which seat, and why only that one
You do not check every seat. You check the extremes, because if the worst seat works, everything between it and the centre works too.
For horizontal sightlines the extreme is the seat at the end of a row: furthest off the centre line, and close enough to the stage that its angle across the opening is acute. The tool checks one row at a time, both ends of it, which is why four numbers are enough and why a tool that asked you to type in a seating plan would be asking for work it does not need.
Start with the front row. In most rooms it is the worst seat horizontally, because it is closest to the stage and the angle is therefore the sharpest, and every row behind it sees more.
Then check the back row if your house widens as it goes back. That rule about the front row holds for a rectangular room and not for every room. A fan-shaped house, a wide balcony or a room with side blocks can put its worst horizontal seat at the back instead: a row 7 meters off centre and 4 meters back and a row 15 meters off centre and 20 meters back are at the same angle to the opening, so both lose the same amount of stage. If the back of your house is much wider than the front, run the numbers twice and design for whichever comes out worse.
That is the reason the field is a row distance rather than a fixed front row. Any row you can measure, you can check.
The four numbers
Opening width is the proscenium opening, or on an open stage the downstage width of the masking. It is the aperture the audience looks through, and it is what the sightlines pivot on: both lines are drawn from a seat through one of its two edges.
Stage depth is how far upstage the stage runs from the setting line, which is the line everything on a theatre plan is measured from and where this drawing puts zero.
Row distance is from the setting line back to the row you are checking. This is the number that moves the answer most, and it is worth trying a meter either way: pushing the audience back is the oldest fix in the book, and it works because moving the seat away from the opening shallows its angle across it.
Row width is that row measured end seat to end seat. The tool puts a seat at each end and works from there, so this is the only thing it needs to know about how the room is laid out. A wide row against a narrow opening is the geometry that loses the most stage, which is why school halls and converted cinemas are hard rooms to design for.
Everything else follows from those four. The two seats sit at half the row width either side of the centre line, the row distance back; the opening edges sit at half the opening width either side, on the setting line; and the lines are drawn between them and continued. There is no fifth number hiding anywhere, which is worth saying because sightline work has a reputation for being harder than it is.
Reading the numbers it gives back
Seen by every seat is the share of the stage floor inside both sightlines. It is a blunt figure and it is meant to be: it tells you quickly whether you are working in a generous room or a mean one. Below about half, the upstage half of your stage is decoration.
Width at the back is how much of the upstage edge every seat still holds. When it reads zero, the sightlines have crossed before they reached the back wall and nothing at the upstage edge is common ground. That is not a fault in the room, it is a fact about it, and the answer is usually to bring the staging downstage rather than to fight it.
Sightlines cross at is the depth where the two lines meet. Upstage of it there is no point on the stage that both end seats can see, which makes it a hard limit on where a moment can play.
What this does not check
This is the plan, so it is the horizontal half. Sightlines have two dimensions and the other one is checked on the section: eye heights, the front row looking up at a raised stage, the balcony looking down over a border set for the stalls, anything low hidden behind anything else. A design with height in it, and most have, needs both. Sightlines, and how to check them covers the vertical check and what to do when a seat loses the moment that matters.
It also assumes a rectangular stage seen through a single opening. On a thrust or in the round the extremes are different seats and the geometry changes shape, though the method does not: find the extreme seats, draw the lines, see what they cross.
And it is a drawing, not a promise. It tells you what the room does. What the design does about it is still yours.
Last updated September 1, 2026