Christus Dominus §39

³⁹ The good of souls requires fitting boundaries not only for dioceses but also for ecclesiastical provinces; indeed it sometimes counsels the establishment of new ecclesiastical regions. Thus the needs of the apostolate will be better met in keeping with social and local circumstances. Thus, too, the relationships of the bishops with each other and with their metropolitans, and with other bishops of the same nation and even between bishops and civil authorities will be rendered easier and more fruitful.