Missa Papae Marcelli (Pope Marcellus' Mass), is a mass by Giovanni Pierluigi da Palestrina. It is his most well-known and most often-performed mass, and is frequently taught in university courses on music. It was always sung at the Papal Coronation Mass (the last being the coronation of Paul VI in 1963). It consists of a Kyrie, Gloria, Credo, Sanctus/Benedictus, and Agnus Dei, though the third part of the Agnus Dei is a separate movement (designated "Agnus II"). The mass is freely composed, not based upon a cantus firmus or parody and perhaps because of this, it is not as thematically consistent as Palestrina's masses based on models. It is primarily a six-voice mass, but voice combinations are varied throughout the piece; Palestrina scores Agnus II for seven voices, and the use of the full forces is reserved for specific climactic portions in the text. It is set primarily in a homorhythmic, declamatory style, with little overlapping of text and a general preference for block chords such that the text can clearly be heard in performance, unlike many polyphonic masses of the 16th century. As in much of Palestrina's contrapuntal work, voices move primarily in stepwise motion, and the voice leading strictly follows the rules of the diatonic modes codified by theorist Gioseffo Zarlino. The third and closing sessions of the Council of Trent were held in 1562–63, at which the use of polyphonic music in the Catholic Church was discussed. Concerns were raised over the use of music that was objectionable, such as secular songs provided with religious lyrics (contrafacta), and whether imitation in polyphonic music obscured the words of the mass, interfering with the listener's devotion. Starting in the late 16th century, a legend began that the second of these points was the impetus behind Palestrina's composition of this mass. It was believed that the simple, declamatory style of Missa Papae Marcelli convinced Cardinal Carlo Borromeo, on hearing, that polyphony could be intelligible, and that music such as Palestrina's was all too beautiful to ban from the Church.
