A lock joint is one way that two perpendicular pieces of wood may be joined together. It is typically achieved by using a special lock joint router bit to cut corresponding grooves into each piece of wood involved. This type of joint is often used in the construction of drawers, where it can provide good tensile strength along the axis at which the drawer is opened. While a lock joint may provide substantially less structural integrity in side-to-side movement, this typically isn't a concern in the construction of drawers. The lock joint may be contrasted with the dovetail joint, which may be stronger while also being more difficult to produce.
Lock joints can be relatively simple to create. After the proper router bit has been acquired and installed, it is usually a simple matter of passing pieces of wood over it correctly. Typically, the pieces that will become the front and back sections of a drawer will be passed horizontally over the router table. This can allow the bit to create the negative space for the side pieces to lock into.
After the front and back have been cut, the same bit can be used on the the side pieces. The difference is that the side pieces are run over the router table vertically, or perpendicular to the surface. This lets the router bit carve hooks that can lock into the grooves on the front and back pieces. In addition to locking together well, these cuts can provide a large surface area for the application of wood glue.
The miter lock joint is a variant that combines mitering with the strength of lock joints. This type of joint involves using a specialized router bit to cut 45° miters on each end of the side, front, and back pieces of the drawer. Like a lock joint, the front and back pieces will have a groove, while the side pieces will have a tongue, so that they can be locked together. This can also provide a good gluing surface, similar to other lock joints.
Lock joints may be considered less aesthetically pleasing than dovetail joints, which are also commonly used in the construction of drawers. Both types of joints can provide good front-to-back strength, while lacking side-to-side integrity. Dovetail joints can be stronger, while lock joints are typically easier to set up and take less time to complete.