Frame joinery is used to assemble structures whose parts are wider than they are deep. This group includes doors, panels or parts of larger furniture, gates and benches, table supports and chairs--anything that can be thought of as being made of "sticks".
From house framing to picture framing, we see screws, nails and bolts being used to fasten wood together. What's wrong with using metal fasteners for everything?
Though it may not sound like it, the biggest bolt or screw still has less holding power than the simplest woodworking joint. The reason: the bolt only "grabs" onto the small portion of wood that surrounds it, and relies on the wood's own strength to hold that portion to the remainder.
But a pair of interlocking shapes which fit into one another, cut directly into the ends of two pieces of wood, will provide many points of contact throughout the wood's mass. And each point of contact will be held together with glue.
Most woodworking glues have plenty of strength; in fact, the glued joint is usually stronger than the wood itself. Two boards that have been joined side-by-side this way can stay glued together for hundreds of years. So why not just butt one board to another and glue our frame together? Why bother cutting the wood into complicacted shapes?
It turns out that the orientation of the grain is critical to the strength of the glued joint. Only side-grain to side-grain joining surfaces can be relied upon to produce a strong glued bond.
This is because the wood's cell structure resembles a bundle of long, thin straws. The end of a board presents only the thin hollowed section of each straw, giving it almost no surface area for the glue to latch onto. So end-grain glued to other end-grain, or end-grain glued to side-grain, will be a very weak joint.
We must rely instead on cutting the wood into shapes that provide for some side-grain to side-grain contact between the two mating pieces--the more the better. That way the glue has something to stick to. If the shapes also interlock, they can give us some mechanical bonding as well.
The open mortise and tenon is the simplest shape: it is nothing more than a slot cut in the end of one board and a reverse slot, or tenon, cut in the other. Without glue, this joint might easily be pulled apart by hand--it has only limited mechanical strength. But the shapes provide two large areas where side-grain wood is glued to other side-grain wood; the glued joint is very strong.
Even stronger is a closed mortise-and-tenon joint, such as the one pictured at right. It totally encloses the tenon inside a matching mortise, while still providing two side-grain gluing surfaces (the sides of the mortise and the faces of the tenon). The mortise is cut into the inside edge of one board, and the tenon is shaped from the end of the mating board. This is the strongest frame joint there is, and a door built using this technique can last for many lifetimes.
A "floating" tenon joint is a simpler version, with mortises cut into both boards. Here, a separate piece is milled to fit both mortises and is partially inserted into each piece.
Dowel joints are yet another variation: wooden dowels are used as tenons, inserted into a series of matching holes drilled in the two boards. The dowels are glued into both pieces, and even though there is minimal side-grain glue surface in the round holes, the joint is surprisingly strong. Using numerous small dowels in place of a few large ones increases its strength even further. But dowel joints still have nowhere near the strength of a mortise-and-tenon joint.
A more recent technique is called biscuit joinery. Basically a floating tenon joint, the joining tenon, or biscuit, is a flat, football-shaped plate of dense hardwood which fits into two mating slots. The biscuits are commercially purchased, and the slots are cut with a special tool which scoops out a half-football slot in each piece.
Plate joinery is useful in plywood construction, but the small number of plates that can be fitted into the joining surface, and the small amount of wood actually held by the plates, limits their strength in frame construction.