Urim v Thummim

(30) Inside the breastpiece of judgement you shall place the Urim and Thummim, so that they are over Aaron’s heart when he comes before HaShem. Thus Aaron shall carry judgement for the Israelites over his heart before HaShem at all times.

Even though a decree of a prophet can be retracted, a decree of the Urim VeTummim cannot be retracted. As it is stated: “By the judgment of the Urim” (Numbers 27:21). Why is it called Urim VeTummim? Urim, which is based on the word or, light, is so called because it illuminates and explains its words. Tummim, which is based on the word tam, completed, is because it fulfills its words.

§ The Sages taught: How does one consult the Urim VeTummim? The one asking stands with his face toward the one who is asked, i.e., the High Priest or the priest anointed for war. And the one who is asked, the High Priest, turns his face toward the Divine Presence, i.e., the Urim VeTummim, in which the explicit name of God is found, by tilting his head downward toward it. The one who asks says his question, e.g.: “Shall I pursue after this troop?” (I Samuel 30:8). And the one who is asked answers him according to the response he receives and says, for example: Thus says God: Go up and succeed. Rabbi Yehuda says: He need not say the words: Thus says God; rather, it is sufficient to relay the content of the response and say: Go up and succeed, since he is obviously only repeating what he was told.