A sight line is what a given audience member can actually see from where they are sitting, and in puppetry it determines how high a stage is built and where every performer can safely stand.
The problem is that it varies
Someone in the front row looks up at a playboardA playboard is the horizontal shelf a stage puppeteer works ...; someone in a back row or a balcony looks down and sees over it. A booth that conceals perfectly from row three can be transparent from the balcony, and the performers will not know until someone tells them.
How it gets checked
Physically. Somebody sits in the extreme seats — furthest, nearest, highest, and hard to each side — while the puppets are worked, and reports what they can see. There is no substitute for doing this in the actual venue.
What it constrains
Playboard height, masking depth, how far upstage a puppet can travel before it is cut off, and how low performers must stay. On tour it gets rechecked in every venue, because a setup that worked last week may not work in a room with a steeper rake.
The camera equivalent
is the frame lineThe frame line is the bottom edge of what the camera sees, a..., which is far easier because there is exactly one viewpoint and you can look at it on a monitorThe monitor is the screen a puppeteer watches while performi....
The compromise
Building tall enough to hide from every seat can put the playboard above what performers can sustain for a whole show. At some point the answer is to accept the highest seats see a little, or to stop selling them.
The hack: photograph the stage from the worst seat before opening. A photo settles arguments that memory and optimism do not.