Hammer-headed key
Also called: hammer head key, hammerhead tenon
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A hammer-headed key is a hardwood peg with a shaped head at each end, resembling the head of a hammer, driven into a curved handrail joint alongside tongues, thin strips of wood set into matching grooves cut in each face, to hold the pieces of the rail together.
Where it is used
Curved handrail sections are built up from several separate pieces rather than cut from one length, to keep short grain, wood grain that runs across a narrow section of timber so that it snaps easily, out of the curve. Those pieces then need joints to hold them together. At the point where the posts meet the head, called the springing, a hammer-headed key tenon is one way to fix the joint; at the crown, the highest point of the curve, the key itself is cut from hardwood with a shaped head worked on each end. More generally in curved handrail work, bolts, false tenons (separate tenon pieces fitted into a mortise rather than cut from the solid timber), hammer-headed keys and bridle joints (joints where the end of one piece is slotted to take a tongue formed on the other) can all serve as the connecting joint.
An alternative fixing
At the springing, the hammer-headed key tenon is not the only choice: handrail screws, screws driven through the joint together with dowels, can fix it instead. Hasluck (1907) called the screw-and-dowel method "the more modern method and equally effectual" as a hammer-headed key and tongues.