The rings of the breastpiece shall be tied to the rings of the ephod with a cord of blue yarn, so that the breastpiece is above the waistband of the ephod and does not swing out from the ephod.

Exodus 25:7
and onyx stones and gemstones to be mounted on the ephod and breastpiece.

Exodus 28:8
And the skillfully woven waistband of the ephod must be of one piece, of the same workmanship--with gold, with blue, purple, and scarlet yarn, and with finely spun linen.

Exodus 28:27
Make two additional gold rings and attach them to the bottom of the two shoulder pieces of the ephod, on its front, near its seam just above its woven waistband.

Exodus 28:29
Whenever Aaron enters the Holy Place, he shall bear the names of the sons of Israel over his heart on the breastpiece of judgment, as a continual reminder before the LORD.

And they shall bind the breastplate by the rings thereof to the rings of the ephod with a lace of blue, that it may be above the curious girdle of the ephod, and that the breastplate be not loosed from the ephod.