chanfrein

Chamfer reaches English through Old French chanfrein or chanfreindre — to cut diagonally — a compound of chant (edge, from Latin canthus) and fraindre (to break, from Latin frangere). The same frangere gives English 'fragment,' 'fracture,' 'infraction,' and 'refrain.' A chamfer is, etymologically, a broken edge: the edge of a piece of timber that has been fractured or cut away at an angle, removing the sharp ninety-degree corner and replacing it with a forty-five-degree slope. The name captures the action with precision: breaking the edge, not rounding it, not squaring it — cutting it at an angle.

A chamfer is distinguished from a bevel by context and purpose. A bevel is an angled surface cut along the full edge of a piece of wood — the blade of a chisel is beveled; a door's edge is beveled to clear the frame. A chamfer is a partial treatment of an edge: it removes the arris — the sharp intersection of two surfaces — and replaces it with a small angled plane. The chamfer typically runs for a portion of an edge's length, stopping short of the ends with a 'stopped chamfer.' The stopped chamfer, with its elegant lamb's-tongue termination where the angled cut returns to the surface, is the joiner's mark of refinement on otherwise plain timber work.

Medieval carpenters chamfered the edges of structural timbers for several simultaneous reasons. Practically, a chamfered edge is less likely to split along its length under impact; the removal of the sharp arris reduces the timber's vulnerability to knocks and blows. Aesthetically, chamfering softens the appearance of heavy beams, giving them a finished quality without adding ornament. The chamfered beam was the standard finish for medieval and Tudor interior woodwork — visible in surviving great halls and church roofs where the principal timbers, often eight or ten inches square, have their lower edges chamfered to lighten their visual weight.

The router has made chamfering trivially easy in modern woodworking. A chamfer bit — a small carbide cutter with a guide bearing — cuts a perfect forty-five-degree chamfer along any edge in seconds. The hand method, using a bench plane or a chamfer plane set to the desired angle, requires marking lines on both faces of the timber and planing until both lines disappear simultaneously — the plane angle is correct when both disappear at the same moment. The visual test is the same one used in 1350: watch the arris and plane until the lines meet. The router does it faster; the plane teaches you what you are actually doing.