Missa «Tu es pastor ovium», 1599
This composition is a typical example of the post-Tridentine polyphonic Mass, the structure of which foresees the development of a Gregorian theme, a contrapuntal elaboration which respects the intelligibility of the text, and a degree of complexity which does not exceed the performance capabilities of average singers. The design adopted by Belli, thus, is that of the so-called Italian missa brevis, in which the Ordinary of the Mass (Kyrie, Gloria, Credo, Sanctus and Agnus Dei) is sung polyphonically, while the Proper (Introit, Gradual, Alleluia, Offertory, Communion) is performed in chant. In the late sixteenth century, each part of the Mass therefore consisted of one or more motets, stylistically linked and sharing a common modal tone. The Kyrie and the Agnus Dei are the shortest parts; both are tripartite, following the structure of the text, and are written in strict imitation. Generally speaking, the last Agnus Dei is composed for a greater number of voices and often contains canons. The Sanctus is subdivided into two sections—the Sanctus and the Benedictus—and they share a common Hosanna. Occasionally it is divided into three sections—Sanctus, Pleni, Benedictus—and the Benedictus is set with a smaller number of voices. The Gloria and the Credo, the longest parts, are composed in numerous sections, divided by strong cadential closes and distinguished by differing numbers of voices, dialogues between vocal groups, and by the alternation between homophonic and contrapuntal episodes.