Two vertical mortises double the grain surface
Mortise walls are end grain, limiting long grain gluing surface
details of craftsmanship when to use
Mortise walls are long grain exposing maximum gluing surface
When basic mortise and tenon joinery won't give you a strong giue joint, take a look at this tried and true variation.
A mortise and tenon joint is a traditional way to join two work-pieces at right angles. It's a strong joint with a lot of gluing surface. But depending on the orientation of the workpieces, there are times when a simple mortise and tenon joint isn't the best choice. If that's the case, using a twin tenon can create a much stronger joint.
HISTORY. The twin tenon has been employed for several centuries. It was first used in ship building, where it was necessary to keep large timbers from twisting or splitting. With side by side tenons in matching mortises, there's less likelihood of the tenoned work-piece twisting or warping. Later furniture craftsmen recognized the value of this joinery and started using it to join workpieces at right angles, such as a rail to a table leg.
STRONGER JOINT. One of the best reasons for using a twin tenon in woodworking is to add gluing sur
face to the joint, since long grain or face grain provides a better gluing surface than porous end grain. In a conventional mortise and tenon joint, like the one shown in the left drawing below, the length of the mortise runs with the grain. This allows for the maximum long grain to long grain gluing surface between the mortise and tenon.
But when I have a project that calls for the face of a rail to lay flat, I know that the long grain
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