Doubled - To give it stability, or to form what was used as a bag for the Urim and Thummim: the latter appears to be the more likely.

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).

Four square it shall be, being doubled,.... That is, when it was doubled; for the length of it, according to Maimonides (c), was a cubit, which is two spans, and so, when it was doubled, was but one, and its length and breadth being alike, as follows:

a span shall be the length thereof, and a span shall be the breadth thereof; a square, which is the measure, the form of the new Jerusalem, the church of Christ, Revelation 21:16 and may denote the perfection, firmness, and immovableness of it, Psalm 125:1. Some have thought that this breastplate was doubled, in order to have something enclosed in it: some imagine, that within this fold were put the Urim and Thummim, which they suppose to be two words engraved on a stone, and different from the twelve stones in it; others, that the name of Jehovah was written and put there, as the Targum of Jonathan and Jarchi on Exodus 28:30 and other Jewish writers, and others, fancy some little images were put within these folds, the name with the teraphim, and supposed to be the Urim and Thummim; but if these were hid in the folds, they could not be seen when consulted; it is most probable there is nothing put within the double, which was not done for any such use; but most likely that it might be strong to bear the weight of the precious stones, put in ouches of gold upon it.

(c) Cele Hamikdash, c. 9. sect. 6.

{i} Foursquare it shall be being doubled; a span shall be the length thereof, and a span shall be the breadth thereof.

(i) The description of the breastplate.

28:15-30 The chief ornament of the high priest, was the breastplate, a rich piece of cloth, curiously worked. The name of each tribe was graven in a precious stone, fixed in the breastplate, to signify how precious, in God's sight, believers are, and how honourable. How small and poor soever the tribe was, it was as a precious stone in the breastplate of the high priest; thus are all the saints dear to Christ, however men esteem them. The high priest had the names of the tribes, both on his shoulders and on his breast, which reminds us of the power and the love with which our Lord Jesus pleads for those that are his. He not only bears them up in his arms with almighty strength, but he carries them in his bosom with tender affection. What comfort is this to us in all our addresses to God! The Urim and Thummim, by which the will of God was made known in doubtful cases, were put in this breastplate. Urim and Thummim signify light and integrity. There are many conjectures what these were; the most probable opinion seems to be, that they were the twelve precious stones in the high priest's breastplate. Now, Christ is our Oracle. By him God, in these last days, makes known himself and his mind to us, Heb 1:1,2; Joh 1:18. He is the true Light, the faithful Witness, the Truth itself, and from him we receive the Spirit of Truth, who leads into all truth.