Suburbicarian diocese
The seven suburbicarian dioceses are Roman Catholic dioceses located in the suburbs that surround Rome. A Cardinal Bishop is named titular bishop of each of the sees, which have varied slightly in history but are now Velletri-Segni, Porto-Santa Rufina, Frascati, Palestrina, Albano, Sabina-Poggio Mirteto, and Ostia; the see of Ostia is conferred on the Dean of the College of Cardinals in addition to his other see; the Dean is chosen by and from the six cardinal bishops of the suburbicarian dioceses, with the approval of the pope (formerly the dean was always the longest-serving cardinal bishop). However, since the 1960s, six of the dioceses have had their own substantive bishop, pursuant to a decree of Pope John XXIII titled Suburbicarii Sedis. Ostia is administered by the Cardinal Vicar-General of Rome.
The titular bishops still formally "take possession" of the dioceses but the substantive bishops exercise all ordinary jurisdiction.