THE ROSARY is made up of two things: mental prayer and vocal prayer. In the Holy Rosary mental prayer is none other than meditation of the chief mysteries of the life, death and glory of Jesus Christ and if His Blessed Mother. Vocal prayer consists in saying fifteen decades of the Hail Mary, each decade headed by an Our Father, while at the same time meditation on and contemplating the fifteen principal virtues which Jesus and Mary practised in the fifteen mysteries of the Holy Rosary.
In the first five decades we must honor the the five Joyous Mysteries and meditate on them; in the second five decades the Sorrowful Mysteries and in the third group of five, the Glorious Mysteries. So the Rosary is a blessed blending of mental and vocal prayer by which we honor and learn to imitate the mysteries and the virtues of the life, death, passion and glory of Jesus and Mary.
From page 17
Nihil Obstat, Gulielmus F. Hughes, S.T.L., Censor Librorium
Imprimatur, Thomas Edmundus Molloy, S.T.D., Archiepiscopus-Episcopus Brooklyniensis, 1954 For more, see The Holy Rosary
St. Louis-Marie Grignon de Montfort's The Secret of the Rosary