quoin
A metal wedge which fits into the space between the type and the edge of a chase, and is tightened to fix the metal type in place.
quoin: wedge that locks type into the chase
A quoin is a tapered metal wedge used in letterpress printing to lock movable type and blocks firmly into a chase, the rectangular steel frame that holds all printing matter during the press run. The quoin wedges into the gap between the type mass and the inner walls of the chase, and a key or lever tightens it to create the clamping force needed to prevent type from shifting or scattering under the pressure and vibration of the printing press.
Quoins come in two main styles: the traditional screw quoin, which uses a threaded screw mechanism turned by hand or key, and the cam quoin, which employs a lever and cam system for faster locking and releasing. Screw quoins are still common in hand presses and small operations where speed matters less than reliability. Cam quoins became standard on larger cylinder and platen presses where makeready had to be quick, since the lever action locks the wedge in a quarter turn rather than several full rotations. Most quoins are made from cast iron or steel and measure from roughly 2 inches to 6 inches across, depending on the chase size.
The name comes from the architectural term for a cornerstone or external angle of a building, a fitting analogy since the quoin sits at the corner of the type form and bears the load like stonework bears weight. In the composing room, the quoin is one of the last elements positioned before the chase goes to press; poor seating or inadequate tightening causes type to shift or fall out during the run, producing blurred or missing impressions and ruining the job. Experienced pressmen develop feel for the proper tightness, since over-tightening can bend soft metal sorts or crack delicate borders.
Quoins must be kept free of dried ink and metal debris, which prevents proper seating and reduces clamping force. The wedge surface and the interior walls of the chase both need to be clean and slightly oiled to allow smooth engagement. Replacement quoins or adjustment shims are part of standard maintenance, since wear on the wedging surfaces gradually reduces holding power over years of use.