Seven of the fourteen compositions comprising Monteverdi's Vespro della Beata Vergine are based on psalm or Magnificat tones, so it is appropriate that this analysis be devoted to one of those pieces. However, with the exception of the two settings of the Magnificat, which are parallel in many respects, the role of the psalm tone in the construction of each psalm differs significantly, and the psalms differ radically from the Magnificats. Therefore, the analysis of no single composition can serve as a model for the others, and each must be addressed separately, with quite different observations to be drawn from each. Of these seven compositions, the six-voice Psalm 121 (Book of Common Prayer 122), ‘Laetatus sum’, is the most complexly organised and the one whose compositional techniques most adumbrate methods of organising concertato psalms in subsequent decades. Rather than the chant serving as an organising force, as in the other psalms and the two Magnificats, the structure of the text is the principal organising element. The cantus firmus is mixed together with repetitive bass patterns, imitative polyphony, chordal homophony, virtuoso solo melismas, duets, trios and falsibordoni. The chant appears only intermittently, either as a solo vocal line or as a single voice in a four- or six-part texture. When it does appear, the chant is usually accorded some prominence either by its position in the texture or by long note values.