Matthew Poole and continuators on Exodus 28:15

15 And thou shalt make the breast-plate of judgement with cunning work, after the work of the ephod thou shalt make it: of gold, of blue, and of purple, and of scarlet, and of fine twined linen shalt thou make it.

This was a square and curiously wrought piece put over the Ephod upon ones breast, called of judgment, because from thence the Israelites were to expect and receive their judgment and the mind of God in all those weighty matters of war or peace, wherein they consulted God for direction.