Because Monteverdi writes modally, he applies most of his compositional genius to surface details rather than to harmonic developments. This means that two things: firstly, we have our ears constantly tickled by his melodic ingenuities and imitative inventions; secondly – and this is where the supercharged moments of the piece occur – when, after a through exploratiuon of one mode Monteverdi makes sudden changes course, we feel the new harmonic area as a visceral, fundamental and powerful shift. When, for instance, Monteverdi shifts abruptly to Bflat major for “Benedicta es” in Audi Coelum we feel the change of mode as a moment of absolute benediction, no matter whether or not we understand the composer’s exploitation of modes.
THere is a great deal of scope for improvisation in this piece, both for singers and instrumentalists. This means that we can perform (and listen to) the work countless times and never hear the same performance twice – meaning that the Vespers always feel alive, vital and on-the-edge. Furthermore, the many moments of direct imitation and echo give the performers plenty of chance to react to and build on each other’s ornamental flourishes as they attempot to delight and even outdo each other.