Miter Joint

A miter joint joins two pieces at an angle — most commonly a 90° corner made from two 45° cuts — with no end grain visible on either piece, just a clean diagonal seam. It's the standard corner for picture frames, molding, and plenty of box construction where the goal is a seamless-looking corner rather than a structural showcase joint.

Why it looks better than it holds

A miter's glue surface is mostly long-grain-to-long-grain at a steep angle, which sounds strong, but the joint has no mechanical interlock at all — nothing physically stops the two pieces from sliding apart under stress except the glue bond itself. Compare that to a mortise and tenon or dovetail, where the joint's shape does real structural work independent of the glue. A plain miter on anything larger than a small frame is genuinely fragile, and it's the reason "just miter the corners" is rarely the whole answer on a real project.

Getting the angle right

Both pieces need the exact same angle for the seam to close cleanly — a fraction of a degree off on a 45° cut becomes a visible gap at the corner, and that error compounds around a four-sided frame. A miter saw with a well-calibrated fence is the standard tool; a table saw with a miter gauge or sled works too, but either way, cut test pieces and check the corner with a square before committing to final-length cuts. Sneaking up on the angle in small adjustments beats trying to nail it on the first pass.

Reinforcing the joint

Because the joint alone isn't mechanically strong, most real miters get help: splines (a thin strip of wood let into a slot cut across the seam, perpendicular to the joint line) add real long-grain glue surface and mechanical resistance to the joint pulling apart; biscuits or dowels do something similar on a smaller scale; and glue blocks or a rabbet cut into the back of the joint add strength on frames where the reinforcement won't show from the front. For anything that will actually be handled or stressed — not just a picture frame hanging on a wall — plan on one of these from the start rather than treating it as an afterthought.

Wood movement across a miter

Because a mitered frame usually runs grain in different directions on each side (a picture frame's top and side rails cross at 90°), the frame as a whole moves unevenly across seasons even though each individual piece is stable along its own length. See the Wood Movement Estimator for how much a given species and width is likely to move, and avoid pinning a mitered frame rigidly on all four corners with something that can't flex slightly with that movement.