Miter Angle For a 10 Sided Box

18° is the miter angle for a regular decagonal (10-sided) box — and it isn't a coincidence that it's exactly half of the 5-sided box's 36° angle, since a decagon's ten corners are, geometrically, twice as many steps around the same circle a pentagon uses.

Why this one specifically

That halving relationship is a genuine shortcut: if you already know the pentagon's 36° by heart (it's the most commonly mis-guessed angle in this whole family, precisely because it isn't an obvious round number like 45° or 30°), the decagon's angle is just that number cut in two. Ten-sided layouts show up in gazebo framing, large planter boxes and decorative bases more often than the odd side counts nearby (9 and 11), because doubling a pentagon's vertex count is a natural move for anyone scaling a design up toward something rounder-looking.

What moves the number

Pure geometry — no species or thickness dependency. What's worth watching is that a 10-sided frame has twenty individual miter cuts closing the loop, more than a pentagon's ten, so the same small setup error has twice as many joints to hide across, or twice as many chances to become visible at the last one.