Choir dress

Choir dress is the traditional vesture of the clerics, seminarians and religious of Christian churches worn for public prayer and the administration of the sacraments except when celebrating or concelebrating the Eucharist. It differs from the vestments worn by the celebrants of the Eucharist, being normally made of fabrics such as wool, cotton or silk, as opposed to the fine brocades used in vestments. It may also be worn by lay assistants such as acolytes and choirs. It was abandoned by most of the Churches which originated in the sixteenth-century Reformation.

Like Eucharistic vestments it derived originally from the formal secular dress of the Roman Empire in the first centuries of the Christian era which survived in church usage after fashion had changed.

Choir dress also differs from "house dress" which is worn outside of a liturgical context (whether in the house or on the street). House dress may be either formal or informal.

Choir dress in the Catholic Church is worn by deacons, priests and bishops when presiding at or celebrating a liturgy that is not the Mass, especially the Liturgy of the Hours. It is also donned when attending Mass without celebrating or observing the Eucharist. It is worn by seminarians, instituted lectors and acolytes, and altar servers and choir members at Mass or other liturgical events.

The basic components of choir dress are:

For seminarians, deacons, and priests the cassock is exactly the same as their normal cassock: a black cassock with black buttons, girded with a black fascia.

Priests who hold additional honors may wear a different cassock: Chaplains of His Holiness wear a black cassock with purple piping, buttons, and fascia, while Honorary Prelates and Protonotaries apostolic wear a purple cassock with scarlet piping and buttons with a purple fascia. A black cassock with amaranth piping and buttons, girded with a purple fascia, serves as pian dress (academic dress) for an honorary prelate or protonotary apostolic. Canons may wear the rochet (if the chapter has been granted usus rochetti by papal indult) with a distinctive mozzetta, the particular colors of which are determined by the chapter.

...
Wikipedia