Allegri's six-part 'Missa Vidi turbam magnam' (I saw a great multitude) was written for performance in the Sistine Chapel. It's an unaccompanied setting because the Sistine Chapel at the time strictly followed the dictate of the Council of Trent that no instrumental accompaniment was allowed. It's a good example of how Allegri who was the foremost exponent of the stile antico had the skill and the flexibility to be able to also compose music in the seconda prattica. In 'Missa Vidi turbam magnam' he uses a far more modern approach to composition than might be expected - there are a lot of modern tonalities and his use of counterpoint is quite restrained. It's performed here with plainchant Introit and Gradual. Enjoy :-).