A papal bull is today the most formal type of public decree or letters patent issued by the Vatican Chancery in the name of the pope.
The closing section consisted of a short "datum" that mentioned the place of issuance, day of the month and year of the pope's pontificate on which issued, and signatures, near which was attached the seal.
For the most solemn bulls, the pope signed the document himself, in which case he used the formula "Ego N. Catholicae Ecclesiae Episcopus" ("I, N., Bishop of the Catholic Church").
In modern times, a member of the Roman Curia signs the document on behalf of the pope, usually the Cardinal Secretary of State, and thus the monogram is omitted.
The term "bulla" derives from the Latin "bullire" ("to boil"), and alludes to the fact that, whether of wax, lead, or gold, the material making the seal had to be melted to soften it for impression.
In 1535, the Florentine engraver Benvenuto Cellini was paid 50 scudi to recreate the metal matrix which would be used to impress the lead bullae of Pope Paul III.
Cellini retained definitive iconographic items like the faces of the two apostles, but he carved them with a much greater attention to detail and artistic sensibility than had previously been in evidence.
Any subject may be treated in a bull, and many were and are, including statutory decrees, episcopal appointments, dispensations, excommunications, apostolic constitutions, canonizations, and convocations.