Picture frame miters are the 45-degree cuts at the corners of the frame. Much of the integrity of the picture frame lies in the quality of the miter itself and the accuracy of the measurement. Most picture frames are joined as discreetly as possible with small nails and glue to keep the frame looking pristine. It's easy to join mitered picture frames together with a few small hand tools.
Lay pieces of the frame out on a flat surface. Apply a small amount of wood glue to the edge of each miter. Bring the miters together. Stretch a nylon strap clamp around the picture frame and tighten the clamp until glue oozes out from each mitered corner. Let the glue dry for one hour.
Loosen the strap clamp. If the frame is 3/4 inch thick, slide the clamp up 1/2 inch around the perimeter of the frame and re-tighten. If the frame is less than 3/4 inch, remove the clamp.
With a pin nailer, shoot a pin nail into each corner underneath the strap clamp if the clamp is still on the frame.
Flip the frame upside down and shoot another nail into the corner from the opposite direction. Remove the clamp.
Use a damp cloth to clean off any excess glue while it is still wet. Chip off dry glue with a chisel. Putty the nail holes with wood putty and sand clean with 100-grit sandpaper.
Always wear safety glasses when using pin nailers.