The Fiori Musicali are a collection of organ pieces for divine service. Frescobaldi, whose custom it was to divide his Fantasias, Ricercari, Canzonas and so on in different books according to category, has in the Fiori, on the contrary, collected pieces of diverse character and arranged them according to the order of the Mass. He offers organists three actual "Organ Masses", i.e., the customary organ interludes from the introductory Toccata to the Postlude after the closing prayer of the Mass. These three Masses are: Missa In dominicis infra annum (Orbis factor), Missa In festis duplicibus I Cunctipotens Genitor Deus) and Missa In festis B. Mariae Virginis (Cum jubilo). The Gregorian melodies used by Frescobaldi are reproduced in modern versions on page 79 of this volume. The various sections which are arranged as themes in the Kyrie and Christe Verses are indicated by small letters. The Fiori musicali were published in 1635 by Vincenti in Venice as "opera duodecima", to our knowledge the only work by Frescobaldi provided with an opus number. The exact title is: Fiori Musicali Di Diverse Compositioni, Toccate, Kirie, Canzoni, Capricci, E Recercari In Partitura A Quattro, Utili per Sonatori, autore Girolamo Frescobaldi, Organista di San Pietro di Roma. Opera Duodecima. Cum privilegio. In Venetia, Appresso Alessandro Vincenti. MDCXXXV. - Preface.