Four square it shall be, [being] doubled
That is, when it was doubled; for the length of it, according to Maimonides F3, 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, ( Psalms 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.
F3 Cele Hamikdash, c. 9. sect. 6.