This is the explanation for using joinery if two pieces of timber, boards or trim meet or collide, they must be fastened together in some way. Some joints or methods of joinery are stronger than others, but some methods of joinery are very difficult to construct. Here is the useful reference.
As there are hundreds or more different wood joinery techniques, woodworkers may also select a technique that matches their skill level.
Some wood joinery methods are much more complex than others, and involve a great deal of expertise in woodworking. Some of the more common ones are included in the joinery techniques mentioned here. The beginner should be able to master these methods with relative ease with a little practise,
THE MITER JOINTT
When two pieces of wood have (one of each) of their ends cut at angles of a certain degree, a mitre joint is used (the total number of degrees for both angles will usually be 90 degrees)…. And by butting the two mitered ends together and fastening them with small trim nails and/or glue, they are joined together.
Often these mitered ends are linked together with a “spline” A spline is a small piece of sharp metal that is inserted into one of the mitered ends and pushed into the end of the other piece of mitered wood, aligning the mitered ends.
In picture framing, mitre joints are widely used, and are the correct way to join two pieces of wood when they form an outside corner, such as trim.
Typical mitre cuts, in particular, are 45 degrees for image frames. The two 45 degree cuts of the mitre will both be 90 degrees in total.