In the course of Old Testament times psalms were composed to express the prayer of God's people in all the situations of their lives. For several centuries before Jesus, the Book of Psalms (the Psalter) was the official collection of songs for the Temple of Jerusalem. Today this book still provides most of the Church's prayers. Almost all the Mass chants are borrowed from it: Introits, Graduals, Tracts, Alleluias, Offertories, Communions. At the end of the book, an index of all the Psalms in the Psalter and of the main Psalms in the rest of the Bible will facilitate finding these texts and identifying their literary form, and will also give suggestions for their use.