The Missa Brevis first appeared in Palestrina’s Third Book of Masses (1570) and in all probability was written about the year 1558. The thematic origin of this mass is not entirely understood. There are indications that it could be based on a theme from Audi Filia, a mass by Claude Joudimal (1510-1572). There are other indications that it could come from the first notes of the Plainsong Credo I, while many parts of the mass are reminiscent of the plainsong mass Cum Jubilo, especially the Sanctus. Whatever the origin the construction of this mass is masterly from start to finish. It is written for four voices, soprano, alto, tenor, and bass. The Benedictus, however, is for three voices, while the second Angus Dei is for five. Particularly characteristic of Palestrina is the purity of the harmonic style, consisting entirely and without exception of the plain triadal progression. The tonality in modern terms is built around F (E flat in this modern transcription), its dominant, subdominant and their relative minor keys. The melodic range within individual sections is restricted to a few notes, showing the direct influence of the Gregorian Chant. An important characteristic of Palestrina expressed clearly in this work is the use of frequent tempo changes, which are governed entirely by the changing meaning and significance of the text. Hence there is a far greater feeling of phrase than of beat and rhythm. The striking unity of form which overwhelms the listener reflects very clearly the peace and serenity that Palestrina himself possessed in his faith. It is not known why the mass came to be called Brevis, for it is not any shorter than the usual four part Palestrina mass.