Dovetail joint
Also called: dovetail, dovetails, through dovetail, lap dovetail, half-blind dovetail, secret mitre dovetail
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A dovetail joint locks two boards together at a corner using a row of angled, wedge-shaped projections, usually called pins, cut into one board and fitted into matching sockets, the recesses cut into the other board to receive them. Because the pins flare rather than running straight-sided, the joint withstands tensile, or pulling, loads better than most other joints used in framing.
What it is and why it holds
The dovetail is one recognised category of framing joint, alongside the bridle joint (two forked, interlocking pieces), the halving joint (each piece notched to half its own thickness) and the housing joint (one piece let into a groove cut in the other); of these, the dovetail is the one built specifically to resist tensile loads. A correctly made dovetail joint is strong and durable, and its strength does not rely on glue or fasteners, which suits it to unfinished natural-wood work. It has long been regarded as a hallmark of fine cabinetmaking. Its long-standing drawback is that cutting it well, particularly by hand, demands time and skill.
Corner and carcase dovetails
In drawer and carcase work, cabinet construction built from panels rather than a timber frame, the plain form is the through dovetail, also called the common dovetail, where the pins run right through the board and show on both faces. Convention places a through dovetail at a drawer's top corner and a lap dovetail, which stops short so it shows on only one face, at the bottom; a lap dovetail is used for drawer fronts and similar fittings. Wearing (2010) argues that putting a lap dovetail at the top offers no real advantage and risks being mistaken for the inferior, machine-made version. Where the joint must not show at all, cabinetmakers use the mitre dovetail, also called the secret dovetail: it hides the end grain, the surface left where wood is cut across its fibres, of both pieces, with the pins and sockets fully concealed within the joint's thickness. In practice only about half the board's thickness is actually dovetailed; the rest is mitred, so the dovetails do not show at the outer corner. Rivington (1887) writes that cabinetmakers reserve it for high-end drawers and boxes, 'when for the sake of appearance it is desirable that the dovetails should not be visible.' Where dovetails must not show on either face, a double-lapped dovetail serves the same purpose as the mitre dovetail. One Japanese variant combines the two ideas: half the socket is a plain square-sided mortise sized to the pin's widest part, and the other half is a standard dovetail socket, with assembly starting by fitting the pin into the mortise before the dovetail socket. In Japanese joinery, joints with a relatively large dovetail pin like this one are classified generally as ari-kake.
Housing and sliding dovetails
Away from corners, the dovetail shape also lets one piece slide into another along a housing, a long groove cut to receive it. Ellis (1908) describes the tapered dovetail housing, in which 'one end of the dovetail is made wider than the other, so that it shall tighten as it goes in.' A sliding dovetail applies the same angled shape to a tongue or tenon so that it slides in endwise rather than being driven in from the side. A slot dovetail is cut across a piece's width rather than its thickness; it is used in carcase construction to attach smaller members to the case, and typically appears in stretcher frames, plinths, the base a piece of furniture stands on, and cornices, a moulding that runs along the top.
Dovetails as reinforcement
The dovetail shape also reinforces other joints without forming a corner in its own right. A mitre joint, where two pieces meet at an angle with only end grain to glue, has a naturally weak glue bond there; a dovetail spline, dovetail key or butterfly key, small dovetail-shaped pieces let into the joint after assembly, strengthens it much as a plain spline or a loose tenon, a separate tongue of wood fitted into matching slots in both pieces rather than cut from one of them, would. On a solid, continuous tabletop, a small dovetail let in at the front and back edges helps stop the ends from curling.
Cutting a dovetail
Hand-cut work begins by scribing the joint's depth with a marking gauge, a tool that scores a line at a fixed distance from an edge. The sockets are then marked by laying the receiving piece flat and positioning the already-cut pin piece against it to trace the pins across; many hand-cutters make a small wooden template to speed this marking. Power-tool methods shortcut hand work: a lap dovetail can be routed with a special attachment and bit, or cut with a dovetailing attachment fitted to an electric drill, while a dovetail jig routs both pieces at once, producing pins and tails of equal width and a rounded profile regardless of the stock's size.