Tongue and groove flooring is designed to be engaged from the start of the installation to the end. For installations that are laid straight in, this is not a challenge since the flooring is manufactured with a side match and an end match. Some design elements require a mitered corner, such as surrounding a fireplace hearth, a return of a bullnose on a stair tread, or in the corners of a border rather than a log cabin configuration. In any of these cases, there is no longer a tongue or groove to lock together.
There are a few ways to lock mitered corners together to prevent individual movement between boards. In a border where several mitered boards need to be locked together, you can run a groove with a router or router table on both sides of the miter and join them up with a slip tongue (spline). This way all the boards will be locked up together as one entire unit. This method can also be applied to individual boards when you are installing a collar around a fireplace hearth.
Another simple way to lock up individual boards is to use a biscuit (plate) joiner. A biscuit joiner cuts a circular slot in the mitered end of the boards. Once the slots have been cut, a biscuit (a semi-circular wood chip cut to a precise thickness to fit the groove) is glued and inserted into the slot. This creates a very tight joint and eliminates any overwood that might occur if the joint were just left unlocked. This is especially important when dealing with factory finished floors as the overwood will not be sanded away.
Biscuits are ideal to use when installing a bullnose that has a return since the finished edge of the bullnose can’t be disturbed. The slot that is cut by the biscuit joiner is invisible once the two pieces are engaged. There are other manufacturers that produce equipment that perform the same task, only instead of cutting a groove for biscuits, these machines cut a mortise where you can insert a tenon. Both systems are very effective at locking mitered pieces together to achieve a flat, smooth surface.