The twelve stones

A description of the priestly garments is mentioned in this week's parsha. THe Choshen Mishpat was a breastplate worn by the Kohen Gadol.

"The Ramba"n gives us an insight into the workings of the Choshen and the Urim V'Tumim: The word Urim means 'lights'. Whenever the Kohen Gadol needed to receive an answer, the letters (of the names of the Tribes, inscribed on the twelve stones of the Breastplate) would illuminate. That was the 'Urim' part. But the letters would be scrambled. It was not as simple as reading out: G-O T-O W-A-R. One needed to break the code -- what were the lights of the letters of the Breastplate saying? The Ramba"n explains that there were secondary holy names called the 'Tumim'. These 'Tumim' gave the Kohen Gadol the ability to interpret the 'Urim'. The ability to use the 'Tumim' to interpret the lights of the 'Urim' was a form of Ruach HaKodesh [Divine Inspiration]."