Outside the Mass there are many vestments and clerical clothing that can be worn by a priest. The cassock is a long black robe that is worn in place of a black suit.
In Catholic countries, priests will wear the cassocks in the marketplace, streets, and other secular public areas. In America, the cassock is usually confined to Church property and specifically worn under vestments, or in some sort of official or liturgical duty.
There are two styles to a diocesan priest’s cassock. First is Roman. Roman style has thirty-three buttons going down the front of the robe signifying the age of our Lord. It may have black piping under the arms to form a holder for a sash. The piping also may outline the sleeves, the button area, and the area around the collar.
Priests wear black piping and sashes; monsignors wear purple piping and sashes; bishops wear magenta piping and sashes; cardinals wear red piping and sashes; and the pope wears a white cassock, piping, and sash. These are called choir robes, and would have been worn when priests would congregate in Church for sung prayers. Monsignors who are prelates of honor, prothonotary apostolic, can wear full purple cassock with piping and sashes. Bishops also can wear full magenta cassocks with piping and sashes, and cardinals, full red cassocks with piping and sashes.
In liturgical services outside of Mass a priest wears a surplice over the cassock. This clerical attire can be made of many materials including lace, but is always white. In addition, a stole and cope are worn over the surplice for solemn services outside of Mass, such as benediction, processions, nuptials, baptisms, and burials. A cope is a long cape that matches the material of the stole. It can be in the liturgical colors of the season.
However, in the ceremony of benediction, an outer garment, known as a humeral veil, is worn over the cope and must always be white. It is worn for the blessing of the Blessed Sacrament because the congregants are receiving Christ’s blessing, not the priest’s. At the Holy Sacrifice of the Mass, if a priest is merely preaching and not concelebrating, he may wear a cassock, surplice, and a stole in the color of the season, which signifies his clerical office.
A mozzetta is a short cape that is worn over a prelate’s cassock and it is usually designed to be elbow length with a round collar. It is fastened by twelve buttons, signifying the twelve apostles. Usually the color of the mozzetta corresponds to the rank of the prelate. However, the Holy Father may wear velvet and ermine trimmed mozzetta along with a pectoral cross suspended by a cord—red with gold for cardinals and green with gold for bishops.
A biretta is a special square cap of three corners and ridges worn by clerics and is similar in usage to the academic hats (mortarboards) worn in the Middle Ages to distinguish rank. A black biretta with black pom-pom is proper for a priest; a black biretta with a purple pom-pom is for a monsignor; a black biretta with a red pompom is for a bishop; a red biretta with no pom-pom is for a cardinal.
The pope does not wear a biretta. Four cornered birettas are worn by clerics with a doctoral degree. The zucchetto is worn under the biretta.