I know about the square triangle 3-4-5, and I know how to use it get a 90° angle.
But I can't think or understand how people use it (accurately) to get 90° on a big plywood sheet.
The most "proper" way that I can think of it mark my reference edge two points 4 units apart, and using a compass or very static (non strechable) line trace two arcs (3 and 5 units) to get the intersection. But I don't have any line that I would trust for that, not any compass large enough (and precise enought to reference from an edge instead of a point in a plane) to do that.
I could have measuring sticks 3 and 5 units in lenght and just join everything to get the needed point, but how to accurately mark this point if you have the sticks on the way?
So, how do you do it properly?