Foursquare it shall be being doubled; a span shall be the length thereof, and a span shall be the breadth thereof. Four-square it shall be - Here we have the exact dimensions of this breastplate, or more properly breast-piece or stomacher. It was a span in length and breadth when doubled, and consequently two spans long one way before it was doubled. Between these doublings, it is supposed, the Urim and Thummim were placed. See Clarke on Exodus 28:30 (note).
Related Commentaries of Exodus 28:16
Exodus 28:16
16 Foursquare it shall be being doubled; a span shall be the length thereof, and a span shall be the breadth thereof.