Here are a few examples of corner joining methods, first below is dovetail keys, useful for deep box frames and wide mouldings, also provides a good, strong joint.
Below is the most common joining method in modern picture framing, it is the underpinner 'V' nail, these little metal right angle nails are fired into the mitre joint using a underpinner machine which drives the nail into the wood, these machines are either manually powered by foot pressure, or pneumatically. These nails are really only intended to hold the joint together while the wood glue sets, the photo below shows the front frame has 5 nails in each corner, this was made by a trainee who thought that more was better, when in fact this number of pins is excessive, wastes money, and even creates a poorer joint. The frame behind shows the 'V' nails on the inside and outside, a much better example.
The example below shows how not to join a frame... looks like an artist has been making his own frames...
Below is a common method of joining large profiles, used extensively in the 19th Century and early 20th Century in England, mitre blocks with nails cross-pinned through the sides of the corners.
Lastly for now (there are other methods I will show later) is another old method, the mitre spline:
