Charles Ellicott Commentary Exodus 28:27-28

"And thou shalt make two rings of gold, and shalt put them on the two shoulder-pieces of the ephod underneath, in the forepart thereof, close by the coupling thereof, above the skilfully woven band of the ephod. And they shall bind the breastplate by the rings thereof unto the rings of the ephod with a lace of blue, that it may be upon the skilfully woven band of the ephod, and that the breastplate be not loosed from the ephod." — Exodus 28:27-28 (ASV)

Two gold rings were also to be sewn onto the ephod, low down and in front, so as to just appear above the "curious girdle of the ephod," and the lower rings of the breastplate were to be laced to these rings by a "lace of blue." The breastplate was thus securely attached to the ephod, and showed above the "curious girdle" without covering it.