Box Joints (Finger Joints): Build the Jig, Dial In the Fit, Cut Perfect Corners

Box joints are the friendly cousin of dovetails: interlocking, strong and handsome, but far easier to cut accurately with a machine. Square fingers knit two boards together like the teeth of a zip, giving a huge area of long-grain glue surface.

The magic is all in one simple jig. Get the jig right, and the joints come out right — like a cookie cutter producing identical shapes all day.

How the jig works

The jig is a fence (on a sled or miter gauge) with a small hardwood key sticking out. The key is exactly as wide as the blade, and sits exactly one blade-width away from it. Cut a notch, lift the board, drop the notch over the key, cut again. Each cut is spaced perfectly from the last.

Building the jig

1. Set up a dado stack (or router bit) to the finger width you want, and set the height to the stock thickness plus a hair.
2. Screw a tall plywood fence to your miter gauge or sled.
3. Cut a notch in the fence and glue in a hardwood key that fits the notch snugly and sticks out about 1".
4. Move the fence so the gap between the key and the blade equals the key width. Use a spare piece of key stock as a spacer.
5. Screw the fence in place and make a test cut.

Mount the fence with slotted screw holes. A tiny adjustment is then just a tap and a re-tighten instead of a whole new jig.

Cutting the joint

Board A

1. Butt the board’s edge against the key and cut the first notch.
2. Lift the board, drop the notch over the key, cut the next notch.
3. Repeat across the board.

Board B (the mating board)

1. Place Board A’s first notch over the key.
2. Butt Board B against Board A’s edge, so B is offset by one finger.
3. Cut B’s first notch, remove A, and continue indexing B over the key.

Dialling in the fit

Test on scrap from the same board. The joint should go together with firm hand pressure.

• Too tight: fingers are too wide — move the key slightly closer to the blade.
• Too loose: fingers are too narrow — move the key slightly away.
• Gaps growing across the joint: the spacing error is accumulating; adjust in tiny amounts.

Router table option

A straight bit in a router table cuts beautifully clean, flat-bottomed notches. Build the same jig on a miter gauge, take light passes in hardwood, and use a backer board to prevent blowout.

Glue-up

Box joints have many glue surfaces, so work quickly. Brush glue onto the cheeks of the fingers, assemble and clamp across both directions. Check the box for square by measuring the diagonals — equal diagonals mean square.

Level up: make the fingers slightly long (1/32" proud) and flush them after the glue dries with a block plane or sander for crisp, perfect corners.

Design ideas

• Contrasting woods: maple and walnut fingers make a striking corner.
• Varied widths: wide fingers in the middle, narrow at the edges — reposition the key for each.
• Proportions: keep fingers roughly as wide as the stock is thick for a balanced look.

Projects that use box joints

Keepsake boxes, drawers for shop cabinets, toy chests and planters. Try them on our toy chest plans, the jewelry box build or a cedar planter. For other options, see our complete joints guide.