The Gemara taught that although the decree of a prophet could be revoked, the decree of the Urim and Thummim could not be revoked, as says, "By the judgment of the Urim." A Baraita explained why the Urim and Thummim were called by those names: The term "Urim" is like the Hebrew word for "lights," and thus it was called "Urim" because it enlightened. The term "Thummim" is like the Hebrew word "tam" meaning "to be complete," and thus it was called "Thummim" because its predictions were fulfilled. The Gemara discussed how they used the Urim and Thummim: Rabbi Johanan said that the letters of the stones in the breastplate stood out to spell out the answer. Resh Lakish said that the letters joined each other to spell words. But the Gemara noted that the Hebrew letter , "tsade", was missing from the list of the 12 tribes of Israel. Rabbi Samuel bar Isaac said that the stones of the breastplate also contained the names of Abraham, Isaac and Jacob. But the Gemara noted that the Hebrew letter , "teth", was also missing. Rav Aha bar Jacob said that they also contained the words: "The tribes of Jeshurun."