Cardinals appointed in pectore are not necessarily informed of their status. Such an appointee cannot function as a cardinal until his appointment is publicly announced, but once announced he enjoys seniority in the College calculated from the time of his appointment rather than from the announcement of that fact.
Popes may choose to keep cardinals' identities secret out of consideration for:
In pectore cardinals are eligible to participate in papal conclaves only if they are publicly named by the Pope before his death. If he does not reveal their names, their cardinalate ceases upon the appointing pontiff's death. Three popes, Benedict XIV, Gregory XVI and Pius IX, were originally created as cardinals in pectore but all were published quite soon afterward.
Among areas where it is believed that in pectore cardinals, whose names were not later revealed, were named include the People's Republic of China and, before the fall of the Soviet Union and collapse of the Iron Curtain, in central and Eastern Europe.
The first Pope to create a cardinal in pectore without later publishing his name was Pope Pius IV, on February 26, 1561. Historians have always speculated about who unpublished in pectore cardinals were, and it is generally believed that this first unpublished in pectore cardinal was Daniele Matteo Alvise Barbaro, whose appointment as a cardinal would have upset the English monarchy and caused hostilities unwanted by the pope.
Although in pectore appointments were not uncommon in the 17th century, all such appointments were published soon after being made until 1699, when Pope Innocent XII reserved two cardinals that were never published. This trend continued until April 26, 1773, when Pope Clement XIV created as many as eleven cardinals in pectore but none were published.
Pope Pius VII created eleven cardinals in pectore; despite the anti-Church hostility of the French Revolution, all of them were eventually published, as were Pope Leo XII's three in pectore appointments.
The outbreak of major revolutions in Europe during the late 1820s, however, caused the proportion of in pectore appointments to all cardinal appointments to rise dramatically: Pope Pius VIII created thirteen cardinals, but only five of them were ever published, whilst Pope Gregory XVI created as many as twenty-eight cardinals (out of a total of eighty) in pectore (of which five were unpublished).
After the Revolutions of 1848 subsided, in pectore appointments declined. Pius IX made only five such appointments out of 123 cardinals (all published within four years of creation) , whilst Pope Leo XIII named only seven cardinals (out of 147) in pectore, of whom all were subsequently revealed.
Pope Pius XI created only one cardinal in pectore, Federico Tedeschini (who was nuncio to Spain just before the Spanish Civil War) in 1933 (published 1935). Neither Pius XI nor Pope Pius XII made any other in pectore appointments, either in European countries affected by the possibility of Marxist revolutions and/or World War II or in any other countries.
With the threat of Communism lingering over Eastern Europe and other parts of the globe, Pope John XXIII made three in pectore appointments on March 28, 1960 and never published them, creating the only case of such an appointment expiring during the twentieth century. It is probable according to many sources that one was Josyf Slipyj, (re-)created cardinal and published by Paul VI in 1965. Pope Paul VI made three in pectore appointments but eventually published all of them, including one (Iuliu Hossu) who died before his appointment was published; the other two were Štěpán Trochta (made cardinal 1969, published 1973, died 1974) and František Tomášek (made cardinal 1976, published 1977, died 1992). (Pope Paul VI appointed Joseph Trinh-nhu-Khuê in pectore in the April 28, 1976 announcement of an upcoming consistory, but published that appointment when the consistory was held on the following May 24.)
Pope John Paul I created no cardinals, whilst Pope John Paul II named four cardinals (of 232 overall) in pectore, of whom all but one were subsequently revealed:
The Italian language version of the phrase – in petto – is also commonly used.