Not too many priests use it in the US but it seems more common in European nations. However, this is the description from the Catholic Encyclopedia. It is not restricted to priests…deacons wear them as well.
A square cap with three ridges or peaks on its upper surface, worn by clerics of all grades from
cardinals downwards. The use of such a cap is prescribed by the rubrics both at solemn Mass and in other ecclesiastical functions. Etymologically, the word
biretta is Italian in origin and would more correctly be written
beretta (cf. however the French
barette and the Spanish
bireta). It probably comes from
birrus, a rough cloak with a hood, from the Greek
pyrros, flame-coloured, and the
birretum may originally have meant the hood. We hear of the birettum in the tenth century, but, like most other questions of costume, the history is extremely perplexed. The wearing of any head-covering, other than hood or cowl, on state occasions within doors seems to have originally been a distinction reserved for the privileged few. The constitutions of Cardinal Ottoboni issued by him for England in 1268 forbid the wearing of caps vulgarly called “coyphae” (cf. the coif of the serjeant-at-law) to clerics, except when on journey. In church and when in the presence of their superiors their heads are to remain uncovered. From the law the higher graduates of the universities were excepted, thus Giovanni d’Andrea, in his gloss on the Clementine Decretals, declares (c. 1320) that at Bologna the insignia of the Doctorate were the
cathedra (chair) and the birettum. At first the birettum was a kind of skull-cap with a small tuft, but it developed into a soft round cap easily indented by the fingers in putting it on and off, and it acquired in this way the rudimentary outline of its present three peaks. We may find such a cap delineated in many drawings of the fifteenth century, one of which, representing university dignitaries at the
Council of Constance, who are described in the accompanying text as birrectati, is here reproduced. The same kind of cap is worn by the
cardinals sitting in
conclave and depicted in the same contemporary series of drawings, as also by preachers addressing the assembly. The privilege of wearing some such head-dress was extended in the course of the sixteenth century to the lower grades of the clergy, and after a while the chief distinction became one of colour, the
cardinals always wearing red birettas, and bishops violet. The shape during the sixteenth and seventeenth centuries was everywhere considerably modified, and, though the question is very complicated, there seems no good reason to reject the identification, proposed by several modern writers, of the old doctor’s birettum with the square college cap, popularly known as the “mortar-board”, of the modern English universities. The college cap and ecclesiastical biretta have probably developed from the same original, but along different lines. Even at the present day birettas vary considerably in shape. Those worn by the French, German, and Spanish clergy as a rule have four peaks instead of three; while Roman custom prescribes that a
cardinal’s biretta should have no tassel. As regards usage in wearing the biretta, the reader must be referred for details to some of the works mentioned in the bibliography. It may be said in general that the biretta is worn in processions and when seated, as also when the priest is performing any act of jurisdiction, e. g. reconciling a convert. It was formerly the rule that a priest should always wear it in giving absolution in confession, and it is probable that the ancient usage which requires an English judge assume the “black cap” in pronouncing sentence of death is of identical origin.
