教皇勅書
Historically a Papal bull was "an Apostolic letter with a leaden seal." A papal bull is an official formal decree. In terms of formality, papal bulls and apostolic constitutions are above encyclicals, followed by apostolic letters. An apostolic constitution is the highest level of decree issued by the Pope, and is generally issued as a bull with the requisite formalities regarding seal and signature. By their nature, apostolic constitutions are addressed to the public, whereas encyclicals are usually addressed to patriarchs, primates, archbishops and bishops in communion with the Holy See.