An excerpt from the third book of the Rationale Divinorum Officiorum of William Durandus of Mende, Englished by T.H. Passmore:
1. After the Stole put on, the Bishop is arrayed in the Tunic, which is otherwise called Subtile, and in the Law, Poderes, that is, the garment which reacheth unto the feet. And this signifieth Perseverance. Thus Joseph is described as having worn such a garment amongst his brethren. For when the other virtues run in the race, Perseverance only receiveth the prize, as it is written, HE THAT PERSEVERETH UNTO THE END, THE SAME SHALL BE SAVED; and One teacheth, BE THOU FAITHFUL UNTO DEATH, AND I WILL GIVE THEE A CROWN OF LIFE. Again, he putteth on the Tunic after the Albe, because by the Albe is figured the chastening of the flesh, while the Tunic is the symbol of those inner virtues, which the perfect need alway to have. The Tunic is not girded, because by reason of its shape it hindereth not the steps: so also the virtues that are set forth by it afford a free walk in the contemplation of God. Yet is the Albe girded, as I have said in the chapter which treateth of the Girdle.
2. In the Old Testament there were two tunics, the tunic of fine linen, and the blue tunic. And at this day also certain Bishops use two Tunics, to show that it beseemeth them to have knowledge of both Testaments, that they may know how to bring out of the Lord's treasure things new and old; or that they may shew themselves that they are both Deacons and Priests. Besides, each of the Tunics doth bear his own proper signification. The white Tunic, which is of fine linen, signifieth Chastity, as may be gathered from what hath been said in the chapter of the Albe. This former Tunic, too, if it be of silk, denoteth Chastity and Humility; for silk taketh its beginning from worms, which are created without intercourse.
3. The second Tunic ought to be blue, as in olden days it was of the colour of the jacinth, which followeth in its hue the sereneness of the sky; wherefore it is a figure of the Saints with their heavenly thoughts and lives, and of celestial thought and conversation. For as that precious stone, the jacinth, doth change his colour with the changes of the sky, being bright when the firmament is bright, and pale beneath the darkling heaven; even so in spiritual figure ought the Bishop to REJOICE WITH THEM THAT DO REJOICE, AND WEEP WITH THEM THAT WEEP. Yet if the Tunic be of any other colour, it beareth accordingly some other meaning.
4. And the Bishop weareth the one Tunic beneath the other, to shew that as that which is hid may not be seen of the people, but is known to the clergy alone, so also that measure of lofty virtues pictured by it, which the perfect man ought ever to possess, may not be open to the eyes of all, but only unto the higher orders, and unto them that are perfect.
5. Furthermore, as touching that which agreeth unto the Head, even Christ, this Vestment in the ancient Priesthood was of blue, and had for its fringe pomegranates and golden bells hanging from its lower hem, that the High Priest might walk in all music; as shall be said in the chapter of the Vestments of the Law. And this giveth an hint of the heavenly doctrine of the Christ, Whereof all have taken knowledge; unto Whom it was said by the Prophet, O THOU WHO TELLEST GOOD TIDINGS TO SION, GET THEE UP INTO THE HIGH MOUNTAIN.
More than all, however, had the Weaver of the doctrine of the Gospel this Tunic, the Wisdom of God, Jesus Christ, and gave it unto His Apostles; for ALL THINGS, said He, WHICH I HAVE HEARD OF MY FATHER, I HAVE MADE KNOWN UNTO YOU. This also was the signification of that Tunic of the Lord, which the soldiers would not rend, for that it was WITHOUT SEAM, WOVEN FROM THE TOP THROUGHOUT - as though they thought the Lord's loss would be great; the which sheweth how great a loss they cause who strive to rend with their heresies the doctrines of the Gospel.
The Subdeacon, also, weareth the Tunic, as shall be shewn in the following chapter.