There is a method I use for large frames using a 3" plain wood scoop.....
I cut a biscuit slot in the corners - toward the inner edge to leave a gap to underpin.
Then I glue all four corners, insert biscuits and locate all the joints. The biscuits are useful
in that they permit lateral movement to align to corners, but hold the joints level - provided
you have cut the slots accurately. Then I throw a webbing strap around the whole thing and cinch
it up, checking the corner alignment as I tighten the strap. In this state the whole frame is rigid and I
can transfer it to the underpinner without disturbing the joints. Put a couple of v-nails toward the back edge.
The pins probably aren't strictly necessary, but I do it anyway.
After that I screw 3" steel 'L' plates in the back near to the inside. I give the plates a bang with a ball-headed
hammer to bend them a little so when they are screwed in they pull the faces of the joint together rather than
pull them apart. Then leave the strap on until the glue dries (overnight?) The frames are mostly about 4' x 3' and
this method helps a lot if you are working on your own. The moulding has a groove along to back which is convenient
for the strap to fit in so it can't slip.
The resultant join is VERY strong. They are for a customer who takes the frames (+pictures) all over the country to
shows. In and out of his car and so forth. They have to be tough.
The only time a corner cracked was when a
freak windstorm blew down the show tent and sent a frame skittering across a field. Even then it was a hairline crack
and I repaired it with ease.
I've tried breaking these joints by hand and even with my enormous strength it's
hardly possible.
I realise this is very much "the way I do it" and it's for frames that are to be finished after joining, but it could equally
be used with pre-finished stuff, taking a bit more care not to mark the moulding.