Biscuit joints are multi purpose joints used by cabinet makers, furniture makers, finish carpenters and I'm sure many other woodworking trades.
Its basic use is to make a butt joint stronger and to help hold the separate pieces of the joint in alignment. These joints must be glued in most applications.
This joint solves one problem that was created with dowel joints. Unlike dowel joints, this joint can be slid sideways for alignment. This makes it faster and more forgiving.
Another joint that it replaces is the loose tenon joint that's hard to make and use when on a jobsite location. With biscuits you just need a portable slot cutter and a handful of biscuits.
Biscuits come in three different sizes numbered 0, 10, and 20, with the smallest being 0. They're used according to the size that fits the application best and are sometimes used in tandom one over the other. When used to keep boards in alignment they're distributed along the length of the board usually 4" to 6" apart.
Most biscuits are made of compressed soft wood chips and when used with glue they swell and make a very tight joint.
The slots for biscuits can be cut into the wood with a router and slot bit designed especially for biscuits or they can be cut with a biscuit slot cutter. These hand held cutters are designed like an angle grinder with a head especially made for cutting biscuit slots. If you use a lot of biscuits you may want to invest in a stationary machine dedicated to cutting slots for biscuits.
Mortise and Tenon Joints
Pocket Hole Joints
Tongue and Groove Joints