Mitre joint
Also called: mitre, miter, mitre joint, compound mitre, keyed mitre, splined mitre
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A mitre joint joins two pieces of wood by cutting the end of each one at an angle, rather than square across, so the two angled faces meet to form a corner. The commonest case is a picture frame or a length of skirting, where each piece is cut at forty five degrees so that together they turn the timber through a right angle.
What counts as a mitre
Ellis (1908) defined the joint broadly: a mitre is a plain joint between two pieces of timber where the surface of the joint lies at any angle other than a right angle, so strictly any oblique joint qualifies, though joiners generally mean the ordinary forty five degree corner unless they say otherwise. As Audels (1923) and Joyce (1987) both describe, a mitre square, kept permanently set at that angle much like a try square with a fixed blade, is used to mark a true mitre, while an adjustable bevel gauge is needed for any other angle. A related tool that Ellis (1908) called the mitre cut carries a rebate, a step cut into one side, to seat a moulding, and is meant for small work.
Why a plain mitre is weak, and how it is reinforced
A plain mitre relies on glue alone, and because each face is an angled cut rather than a run of long grain meeting long grain, that glue line is not especially strong. Joiners have reinforced it in several ways for as long as records survive. Rivington (1875), whose wording reappears almost unchanged in an 1887 edition of the same book, describes keying the joint: thin slips of hard wood, coated in glue, are let into the corner across the joint line for extra strength. The same source records how the joint behaves as timber shrinks across its width: the shrinkage does not open the outer angle of the mitre, but the inner angle does open slightly. In the twentieth century, cutters that interlock the two mitred faces mechanically became available: Hylton (2005) described the lock mitre, cut with a shaped router bit and originally worked out as a table saw setup, which grips the two pieces together in a way that a plain glued mitre cannot. Joyce (1987) rated the secret mitre dovetail, a mitre that conceals a row of dovetails within the joint so that neither the mitre line nor the dovetails show from outside, as the strongest form the joint takes.
Cutting a mitre
By hand, the simplest guide, as Hasluck (1907) describes, is a mitre block: a piece of wood with saw kerfs cut through it at the required angle, into which the work is laid and sawn. Hasluck (1907) also noted that mitre blocks come in inclined and other patterns for less common angles. A mitre box does the same job inside a closed frame; Audels (1923) thought a good workman would rarely need to plane a mitre to fit afterwards, since careful sawing in a box gets the joint close enough to invisible once it is glued or nailed home, and reckoned the plain wooden box would always have its place, not because it works better than the patent versions but because it stands up to careless handling without being easily broken. Rogowski (2002) set hand work against machine work: a backsaw guided by eye, working freehand or against a box, can get a mitre close, but making it fit exactly this way still usually calls for some final paring, whereas a dedicated mitre saw repeats the same angled cut time after time. On a table saw, as Duginske (1999) explains, a plain mitre is cut with the table itself set square and only the mitre gauge, the sliding fence that guides the timber past the blade, angled to the required corner; a compound mitre, needed wherever a piece has sloping as well as angled sides, is cut with both the mitre gauge and the table angled together. Noll (2002) pointed out that the exact slope for a compound mitre is rarely critical, since an approximate angle at both saw and blade will still let the piece fit together, though any slight misalignment at the glue up stage will still throw the joinery out.