While I wish I were able to give a more definitive answer, the Bible does not say much about these parts of the high priestly equipment. What we can know is somewhat helpful, leaving other matters as unexplained/unrevealed matters of wonderment.

• “And Aaron shall bear the names of the children of Israel in the breastplate of judgment upon his heart, when he goeth in unto the holy place, for a memorial before Jehovah continually. And thou shalt put in the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron’s heart, when he goeth in before Jehovah: and Aaron shall bear the judgment of the children of Israel upon his heart before Jehovah continually” (Exod. 28:29-30).
• “And he placed the breastplate upon him: and in the breastplate he put the Urim and the Thummim” (Lev. 8:8).
• “And of Levi he said, Thy Thummim and thy Urim are with thy godly one, Whom thou didst prove at Massah, With whom thou didst strive at the waters of Meribah” (Deut. 33:8).
• “And the governor said unto them, that they should not eat of the most holy things, till there stood up a priest with Urim and with Thummim” (Ezra 2:63).
• “And the governor said unto them, that they should not eat of the most holy things, till there stood up a priest with Urim and with Thummim” (Neh. 7:65). [This last reference in biblical history implies their absence.]
• The Urim alone is mentioned in Numbers 27:21 and 1 Samuel 28:6, possibly being used as a synecdoche (i.e., a figure of speech where a part is used for the whole).

Surprisingly, the Jews had no clear understanding of their meaning but were divided in their opinions regarding the Urim and Thummim. Some thought them to be like dice used in the casting of lots, while others thought them to be jewels making up the high priestly breastplate. Some felt that Thummim gave a “Yes” answer while Urim supplied a “No,” with each object having a Urim side and a Thummim side. In this event, two Urims would have been needed for a “No” answer, and two Thummims for a “Yes.” If one of each turned up, then there was no answer from the Lord.

I believe that we can conclude the following from the passages: This equipment was associated with the judgment of the nation as determined by the High Priest. They must have been employed by the High Priest in inquiring of God about matters of His will. There were times when God gave no answer by this means, even when asked. This method evidently stood on par with prophets and dreams, which were authorized avenues for God’s revealing of His will.

- edited, Truth Magazine, December 2019