The most distinctive characteristic of a bull was the metal seal ("bulla"), which was usually made of lead, but on very solemn occasions was made of gold, as those on Byzantine imperial instruments often were (see Golden Bull). On the obverse it depicted, originally somewhat crudely, the early Fathers of the Church of Rome, the Apostles Saint Peter and Saint Paul, identified by the letters "Sanctus PAulus" and "Sanctus PEtrus" (thus, SPA •SPE or SPASPE). St. Paul, on the left, was shown with flowing hair and a long pointed beard composed of curved lines, while St. Peter, on the right, was shown with curly hair and a shorter beard made of dome-shaped "globetti" (beads in relief). Each head was surrounded by a circle of "globetti", and the rim of the seal was surrounded by an additional ring of such beads, while the heads themselves were separated by a depiction of a cross. On the reverse was the name of the issuing pope in the nominative Latin form, with the letters "PP", for "" ("Shepherd of Shepherds"). This disc was then attached to the document either by cords of hemp, in the case of letters of justice and executory letters, or by red and yellow silk, in the case of letters of grace, that was looped through slits in the vellum of the document. 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.