@Optionparty Those can be pretty handy. Specially the really memorable 3 - 4 - 5 one.
I have used that in the past to build a perfect 90 degrees corner in a building. A good tape measure, some lumber or metal lengths, and you get a square angle no sweat.
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@design_RG Matching up two diagonals of a square works well too.
@Optionparty I love geometry.
We can even apply angle bissection via arcs in a construction setting and get accurate results.
String and a nail, swing a big arc, doig again from other points, intersection is the point we need.
A rigid piece like a metal profile is nice too, avoiding the possible error by stretching in the string.