Corner block joints are intermediaries. Instead of connecting two boards directly together, each board is connected to the corner block instead.
The corner block does not offer any structural improvements over other joints, however it does offer some design possibilities that might otherwise be unavailable, especially if the boards being joined together are manufactured woods such as plywood or veneered, where you might wish to hide the edges of the boards. The corner block can be molded or shaped, can be made of a different material or color than the main boards, or can even be a different thickness than the boards which are connected to it, possibly producing a proud corner.
The most common material for the corner block is solid wood. Though manufactured wood can be used, the shaping and stylistic benefits of corner blocks are lost since the surface of the block would still need to be covered in some way.
If a solid wood corner block is used, the proper orientation of the grain, relative to the case sides, is crucial. Misaligning the grain directions can cause joint separation and failure, and may even damage the wood of the case sides.
If the case side panels are made from manufactured material (such as plywood, MDF or particleboard), the block's grain should be aligned parallel to the direction of the board's edge (regardless of the direction of the grain on the surface veneer on plywood). This is because manufactured boards will not experience any significant movement due to seasonal moisture content fluctuations, but the hardwood corner block will. By aligning the corner block's grain to run along the edge of the case panel, the expansion will not run along the length of the panel, and so it will have no effect. If the grain were to run in any other direction (out from the edge of the panel instead of along the length of the edge), the seasonal movement of the corner block would make it wider or narrower than the panel it is joined to, and the joint would fail.
If the case side panels are made from solid wood, the grain of the corner block must run in the same direction as the grain in the panels. Aligning the corner block in any other direction will cause the joint to fail due to seasonal wood movement. Note that this also means that the grain on the two panels being joined (through the corner block) must also have their grain running in the same direction.
The tongue can be formed on either the side panel or the corner block, but it is often easiest to place the tongue on the corner block since forming details on the ends of long side panels can be difficult. If the corner block is used to join solid wood panels, it is recommended that the grain on the corner block run diagonally, from one tongue to the other. This is to provide maximal strength to each tongue so it can better resist the shearing force once it is mated with the panels. If the corner block is being used with manufactured boards, the block's grain is running parallel to the edge of the panel and this is not a concern.
A full-length, stopped, or blind spline can be used to attach the corner block to the side panel. This eliminates the complexity of forming tongues on the narrow corner blocks, and it can also be accomplished with just one setup of the tablesaw or router table.
Instead of using long splines, biscuits can be easily substituted.