For all that, it is practically certain that no uniform rules can have been followed as to superscription, formula of salutation, conclusion, or signature.
It was only when some sort of registry was organized, and copies of earlier official correspondence became available, that a tradition gradually grew up of certain customary forms that ought not to be departed from.
In the earlier papal letters, however, there are as yet but few signs of the observance of traditional forms.
A much more precise acceptance has prevailed since the fifteenth century, and a bull has long stood in sharp contrast with certain other forms of papal documents.
For practical purposes a bull may be conveniently defined to be "an Apostolic letter with a leaden seal," to which one may add that in its superscription the pope invariably takes the title of In official language papal documents have at all times been called by various names, more or less descriptive of their character.
Gregory's successors followed him in this preference, it was not until the ninth century that the phrase came to be used invariably in documents of moment.
Before Pope Adeodatus (elected 672) few salutations were found, but he used the form "salutatem a Deo et benedictionem nostram." The now consecrated phrase "salutatem et apostolicam benedictionem" hardly ever occurs before the tenth century.
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