Unlike the chasuble which we’ve mentioned here, the mitre is not part of classical Roman costume.
“One theory is that the mitre developed out of the conical helmet of white linen first worn in processions out of doors by the Roman pontiff in the eigth century. This helmet was known as the Frigium or Phrygian Cap. By the opening of the twelth century a dent or crease had developed in this cap. In the late twelfth or early thirteenth century the cap was turned round the other way and began to assume the form which we now commonly associate with episcopal head-dress. During the next four centuries, the mitre increased in height until we reach the towering form of the later Renaissance.
As early as the time of Pope Gregory X (AD 1271) a distinction is drawn between the kinds of mitres and the occasions of their use. The modern Roman ceremonial reflects these distinctions by recognizing three kinds of mitre: (I) mitra pretiosa (jewelled), (2) mitra aurifrigiata (without jewels and used at times of less solemnity), (3) mitra simplex (plain with white linen).” -Liturgical Vesture
The shape of the modern mitre is also steeped in great symbolism, as this prayer from the consecration of a bishop (from the old Pontificale Romanum) indicates (translation mine):
|Imponimus, Domine, capiti hujus Antistitis et agonistae tui galeam munitionis et salutis, quatenus decorata facie, et armato capite, cornibus utriusque Testamenti terribilis appareat adversariis veritatis; et, te ei largiente gratiam, impugnator eorum robustus exsistat, qui Moysi famuli tui faciem ex tui sermonis consortio decoratam, lucidissimis tuae claritatis ac veritatis cornibus insignisti: et capiti Aaron Pontificis tui tiaram imponi jussisti. Per Christum Dominum nostrum.||We place, O Lord, the helmet of your fortification and salvation, upon the head of this Bishop, your combatant, with beauty adorned, and head armored, so that with the horns of each Testament he might appear terrible to the adversaries of truth; and, with You having lavished grace upon him, that he stand out as their valiant attacker, You who marked the face of Your servant Moses, embellished from the partnership of Your word, with the most shining horns (trumpets) of your clarity and truth: and You ordered a crown be placed (lit. ‘Phrygian bonnet w/ lappets) on the head of Aaron your high priest. Through Christ Our Lord.|
The mitre is also reminiscent of the mitznefet, the head-dress of the Temple High Priest, which has been translated as “mitre”.
The Phrygian Cap, possible forerunner of the mitre
The High Priest with the Mitznefet