Mitre joints: accurate angles, equal lengths and reinforcement

A mitre joins two boards with angled cuts so the corner shows no end grain. It is the joint of picture frames, casings, mitred boxes and many cabinet face details. It looks simple and is unforgiving: small angle errors show as gaps, and the glue surface is mostly end grain, which is weak on its own.

The angle

For a closed frame with equal sides, each mitre is half the frame’s corner angle:

mitre angle = 180° ÷ number of sides

Sides	Mitre angle
4 (square or rectangle)	45°
6 (hexagon)	30°
8 (octagon)	22.5°
12	15°

On most mitre saws and mitre gauges, 0 degrees is a square crosscut, so these are the numbers to set on the scale.

Why small errors multiply

A four-sided frame has eight mitre cuts. If the saw cuts 45.5 degrees instead of 45, each corner is out by a full degree, and the error across the frame adds up to 4 degrees, which opens as a visible gap at the last corner. The same is true of length: if opposite sides differ by 1/32 inch, the frame is not a rectangle and the corners will not close.

• Test the setting. Cut four pieces of scrap and assemble a test frame. Or cut two pieces, put them together and check the corner with a reliable square; any error shows doubled.
• Cut matching parts against a stop. Opposite sides must be identical. Cut them in the same setup, one after the other.
• Sneak up on length. Cut slightly long, check, then trim the final amount.

Measuring length

Frame parts are usually specified by their length at the long point, the outside tip of the mitre. For a picture frame, the inside dimension must also allow for the rabbet that holds the glass and backing. Decide which edge you measure and use it consistently.

Reinforcement

A glued mitre joins end grain to end grain, or nearly so, and end grain absorbs glue rather than bonding well. Most mitres benefit from reinforcement:

• Splines: a thin strip of wood in matching grooves cut across the mitre faces, running the length of the joint. Good for boxes and wide frames.
• Keys (feathers): thin slips glued into saw kerfs cut across the outside corner after glue-up. Strong and decorative.
• Biscuits or loose tenons: placed in slots across the joint; they also help alignment during glue-up.
• Pins or brads: hold small mouldings while glue dries.

Size the reinforcement to the stock. A spline or key about a third of the stock thickness is a common starting point.

Wide mitres and wood movement

A mitred corner of wide solid boards has a problem. Each board changes width with the seasons but not length, so the mitre angle on each piece changes slightly. When the boards shrink, the joint tends to open at the inside corner; when they swell, it opens at the outside. On narrow frame stock this is barely visible. On wide solid boards, such as a mitred apron or a wide frame around a panel, it shows every year. Plywood and veneered stock do not have this problem. See wood movement.

Gluing up

• Size the end grain first with a thin coat of glue, let it soak in for a few minutes, then apply a second coat and assemble.
• Use band clamps, corner clamps or clamping blocks glued to the outside so pressure is square to the joint rather than sliding it.
• Check the frame for square by measuring its diagonals before the glue sets.

In GrainDraft

Mitre is one of GrainDraft’s eight joint families. Complementary bevels meet at the connection, and the joint preview lets you inspect the meeting faces and the stock that remains. Because opposite parts can share one named dimension, a frame’s sides stay equal when you resize it.

Related: half-lap joints, reading a shop drawing.