D-Hooks are mounted on the back, sides of a picture frame, between which framing cord, or framing wire is strung.

D-Hooks come in a large variety of shapes and sizes, and the choice on whcih to use is generally based on the weight of the framed opbject, or the framing material. 

D-Hooks can be purchased in multi packs from hardware stores as well as any Framing Supplies.