Under legislation of Pope Pius X, vicars general and vicars capitular (the latter are now called diocesan administrators) are titular (not actual) Protonotaries durante munere, i.e. as long as they hold those offices, and so are entitled to be addressed as Monsignor.
The only privileges of dress that Pope Pius X granted them were a black silk fringed sash, black piping on the biretta with a black tuft, and a black mantelletta. As a result of this they were in some countries referred to as "black protonotaries." However, "Pontificalis domus" of Paul VI removed this position from the Pontifical Household, even though the title of "monsignor", which is to be distinguished from a prelatial rank, has not been withdrawn from vicars general, as can be seen, for instance, from the placing of the abbreviated title "Mons." before the name of every member of the secular (diocesan) clergy listed as a vicar general in the Annuario Pontificio. (Honorary titles such as that of "Monsignor" are not considered appropriate for religious.)
The Secretariat of State has set minimum qualifications of age and priesthood for the appointment of Chaplains of His Holiness (35 years of age and 10 of priesthood), Honorary Prelates (45 of age and 15 of priesthood) and Protonotaries Apostolic Supernumerary (55 of age and 20 of priesthood). However, it waives the minimum age limit for vicars general proposed for appointment as Honorary Prelates, in view of the fact that, as long as they hold the office of vicar general, they also hold the still higher rank of Protonotary Apostolic Supernumerary. For the same reason, the Secretariat of State does not consider it appropriate that someone who is already a vicar general be appointed only a Chaplain of His Holiness.
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Other articles related to "monsignori":
... proprio Pontificalis Domus of 28 March 1968 simplified the classification of monsignori or lesser prelates ... As a result monsignori are now classed into the following three ranks, in descending order of precedence Apostolic Protonotary, of which two types are ...