Tetzaveh 

תצוה

The High Priest always wore the breast piece of judgment on his chest. There were twelve precious stones embedded in the breast piece, and each was inscribed with the name of one of the tribes. These items were the Urim and the Tumim, through which it was possible to ask questions of God and receive a response from God, as explained by the Sages. 

The Sages taught: How do they ask questions of God with the help of the breast piece? The questioner faces the questioned [i.e., the High Priest], and the questioned faces the Divine Presence present in the Urim and the Tumim…. One does not ask aloud and does not contemplate in his heart. Rather, he asks in the manner that Hannah spoke in her prayer [by forming the words with his mouth, but without saying them out loud]…. One does not ask about two matters together, and if he asked about two matters, God responds to only one…. Why are they called the Urim and the Tumim? Urim – because they illuminate [me’irin] their words. Tumim – because they complete [tam] their words…. How is it accomplished?<footnote data-num="19"/> Rabbi Yohanan says: The letters that provide the answer protrude. Reish Lakish says: The letters that provide the answer connect to each other. <source>(Yoma 72a; Shemot Rabba 38) </source>

The High Priest always wore the breast piece of judgment on his chest. There were twelve precious stones embedded in the breast piece, and each was inscribed with the name of one of the tribes. These items were the Urim and the Tumim, through which it was possible to ask questions of God and receive