Miter Angle For a 9 Sided Box

A regular 9-sided box needs a 20° miter at each end — a clean, whole-number angle, which makes it the odd one out in a strange way: it's the "nicest" number in this whole part of the polygon sequence, yet almost no miter saw has a built-in stop for it.

Why this one specifically

Saw makers put positive-stop detents at the angles people actually use — 15°, 22.5°, 30°, 45° — because those come from the side counts (12, 8, 6, 4) that dominate real furniture and construction work. A 9-sided shape is rare enough in practice that 20° never made that list, despite being a rounder number than the 22.5° an octagon needs. The practical result is backwards from what you'd expect: cutting a nonagon means reading the scale and setting it by hand every time, the exact thing the "nicer" round number would seem to make unnecessary.

What moves the number

This is fixed geometry with no material dependency. The only thing worth double-checking is that your saw's scale actually reads accurately at 20° specifically — a scale can be dead-on at its marked detents and still drift a little at angles between them, since those in-between positions get far less real-world use and scrutiny.