The carved clusters at the four corners of this frame look decorative. They are actually the solution to a problem.
A running band of repeating ornament works beautifully along a straight length. At a corner it stops working, because the pattern has to turn ninety degrees. Take it round the corner and one repeat has to bend through the turn, which distorts it. Mitre it and the pattern is cut through at forty-five degrees, leaving two half-motifs meeting nose to nose - and on a gilded frame at eye level, that is exactly what people notice.
The old answer, and ours, is not to try. A carved cluster is placed over each corner, and the running bands simply stop against it on both sides. The pattern never has to turn.
That also frees the run itself. Because each side ends at a cluster rather than at a mitre, the repeat can be adjusted a fraction to divide evenly into the length of that side - so no partial motif appears anywhere, and the two long sides carry the same count.
One ornament, doing three jobs: covering the joint, ending the pattern, and letting the repeat come out right.