Grooves, dados and rabbets
These three joints are all channels or steps cut into one board to receive another.
- A groove runs along the grain of the board, in the middle of the face.
- A dado runs across the grain, also in the face of the board.
- A rabbet is a step cut along an edge or end, leaving a ledge, so it has only two walls instead of three.
The groove and dado are channels with a floor and two walls. The rabbet is an L-shaped notch at the edge of a board. In some regions, the terms overlap, and people call any such channel a “dado”, but the grain direction is the usual distinction.
Where each is used
Dados hold shelves and dividers in bookcases and cabinets. The ends of the shelf sit in the channel, which supports weight and keeps the shelf from sliding. This is a big improvement over a plain butt joint, since the shoulders bear the load and the glue surface is larger.
Grooves hold panels in frames, drawer bottoms in drawer sides, and thin back panels. Often the panel is not glued in, so it can move with humidity changes. Considering wood movement, a solid panel set in a groove usually has a bit of room to expand.
Rabbets are common at box corners, along the back of a cabinet where a back panel fits, and along the edges of drawer fronts. A rabbeted corner hides part of the end grain and gives a ledge that helps align parts during glue-up. It resists racking only moderately by itself, so it is often reinforced with nails, screws, or dowels.
A note on plywood
Sheet goods such as plywood often have a thickness slightly different from the nominal figure, so a dado cut to a stated size may fit loosely or tightly. Measuring the actual material and testing the cut on scrap helps. Some people cut dados in two passes or use a router bit chosen for the real thickness. Plywood also tends to tear at the edges of cuts, which a scoring pass or a sharp blade can reduce.
Layout
Mark the position of the dado by holding the mating piece against the board and scribing its edges, which avoids measurement mistakes. Use a marking knife for crisp walls and a combination square for lines square to the edge. For a series of identical dados in matched sides, clamp the parts together and mark across both, so shelves stay level.
A dado near the middle of a long board weakens it slightly, so most are no deeper than about one third of the board’s thickness. Treat that as a rough guide rather than a rule.
Tools that cut them
Table saw. A standard blade makes repeated passes to clear the waste, or a stacked dado set cuts the full width at once. Many dado sets let you adjust the width with shims. The workpiece is guided by the fence or a miter gauge. A table saw can be dangerous; keep guards where possible, use push sticks, and read the manual, since not every saw can accept a wide dado stack.
Router. A straight bit with a fence or clamped straightedge guide cuts a clean channel, and a router table suits narrower parts. A benchtop router in a small table works well for small work. Cut in several shallow passes and keep the router moving against the direction of rotation for feed. Choose bit sizes suited to the stock, or use an undersized bit in two passes to fit exactly.
Hand saw and chisel. Saw the two walls, remove the waste with a chisel, and level the floor with a router plane if you have one. This is slow, but it works on a single piece in a place where setting up a machine would not be worth it.
Rabbet plane or shoulder plane. A dedicated plane cuts a rabbet along an edge, guided by a fence and depth stop. It is quiet and produces little dust.
Circular saw. Some people make repeated kerfs with a depth stop and clear the waste with a chisel. It is rough, and it needs care, but it can be useful on site.
Getting a good fit
The channel should be wide enough that the mating piece slides in with light hand pressure, with no gap showing. A loose fit weakens the joint and looks poor. If a dado is too tight, a few passes of a plane or a light sanding of the end of the shelf is usually better than forcing it, since that can split the sides.
Safety
Wear eye and ear protection, and use dust collection or a respirator-type mask when machining sheet goods, which raise fine dust. Clamp the workpiece, keep hands out of the cutter path, and unplug the tool or remove the battery before changing blades or bits. Follow your tool’s manual. This is general information, not professional advice.