This is a picture of a Z-Square.
It is very useful for building 3 dimensional frames.
You can make them any size you like.

To assemble them, just clamp each of the 3 sides to a layout table, as you go.
As long as all 3 sides are square, the whole thing is square.

This is a picture of a tiny one I made for TIG welding tube frames.
I have made them up to 8 feet tall.

The big ones are a pain to store unless you have a hook in a corner 
of your ceiling.

Someday I will tackle making a folding one.