Miter Angle For a 3 Sided Box

A 3-sided box needs a 60° miter at each end — the steepest cut in the common flat-frame set, and the one where a small setup error costs the most.

Why this one specifically

Three-sided frames are genuinely rare in furniture and joinery — most projects that could be triangular end up as some other shape instead — but the ones that do exist, like triangular plant stands or trivets, are the least forgiving of the whole polygon family for a structural reason, not an aesthetic one: with only three corners in the loop, a miter error at one joint has just two other corners to be absorbed across before the shape has to close, so the same error that would be invisible on a 12-sided ring shows up as an obvious gap here.

What moves the number

Nothing about the material changes this angle — it's pure geometry, fixed by the side count alone. Cutting a test corner from scrap and checking it against a square before committing real stock matters more here than on any other side count this site covers, precisely because of how little room three corners leave to average out an error.