This miter joint is close to fitting, but it has a slight gap on the inside of the corner. Few things diminish a good wood project like miters with visible gaps. It is a great feeling to make a picture frame or install molding and have tight, clean miters.
Step 1: A Quick and Easy Solution to Miter Joint Gaps
Firmly clamp the members of the joint at the angle needed, in this case a 90 degree corner. Gently run a fine tooth saw down the joint between the two pieces. A back saw or a dovetail saw works perfectly. When finished, slide the members of the joint together and fasten in place with glue or nails.
Step 2: The Final Fit
This is the same joint after running a dovetail saw through it by hand. The fit is tight. The look is perfect and very professional.
If your gap is large, bring the two members of the joint together and run the saw between them again. In most cases, you will not create a fit problem for the other joints by removing a tiny bit more material at this corner.
If you are working at installing molding in a house, be aware that the corners on many walls will often be a bit greater or lesser than a perfect 90 degrees. Take a couple of pieces of 1 x 2 two or three feet long that are very straight. Fasten them together at one of their ends with a bolt and wing nut. Use this as an angle finder to check the actual angles of your corners. Measure the opening in the angle finder with a protractor. Adjust your miter saw accordingly. If there are still inaccuracies in your cuts, you will have less material to remove with a dovetail saw in order to get rid of any gapping.
This idea originally came from a high school woodworking textbook published in 1910. I believe it was called "Woodworking for Secondary Schools."