The best quality frame that a framer can show you is a closed corner frame.  It is so called because there is no corner miter or corner seam.

Most custom picture frames are created by cutting and joining what is known as length moulding, in the form of 8 to 10 foot stick lengths all pre-finished from the manufacturer. The frame is then made to size by cutting (mitering) the four sides of the moulding to length and then joining it in a 90 degree vise by gluing, v-nailing or thumb-nailing the corners together.  If there is a finish variation, or a pattern to the frame, the pattern in the corner will probably not match.  Although the frame is joined, you can still see the corner seam  The framer now uses various types of putty and creams to "heal" the miter joint until he is satisfied with the appearance of the joint.

With custom closed corner picture frames the frame builder joins the frame with raw milled wood, and then carves in the design or adds appliques (called compo) to embellish the frame. This yields a corner with continuity of the pattern, and when finished will not show the miter mark in the corner. The design pattern is symmetrical on the picture frame and all four frame corners have the same balanced pattern. Finishing in the form of synthetic or genuine gold or silver leaf, painting, staining, and antiquing is now applied, sometimes in stages over a period of several days.

It is mostly true, but not always, that a closed corner frame is more expensive than a cut and join frame.  Our shop has many closed corner frames that we can sell competitively with pre-finished length moulding frames, so it is always wise to inquire.
And the end product is always worth the additional consideration