The Book of Psalms for liturgical as well as private devotional use.
The psalter is the Book of Psalms. Medieval manuscripts of the Psalms were used in liturgical as well as private devotional contexts and often contained ancillary texts such as a calendar, Canticles, creeds, a litany of the saints, and prayers. Psalters designed for use in the performance of the Divine Office often contain other relevant texts, such as the Hours of the Virgin. The psalter was the principal book for private devotions before the emergence of the Book of Hours in the thirteenth century. The Psalms also formed a major part of many medieval prayer books from the ninth century on. In the non-monastic Roman liturgy of the Middle Ages, all one hundred and fifty Psalms were recited each week, the majority at matins and vespers. The cycle began at matins on Sunday with Psalm 1 and continued at matins on the following days: Psalm 26 was the first recited on Monday, Psalm 38 the first on Tuesday, Psalm 52 the first on Wednesday, Psalm 68 the first on Thursday, Psalm 80 the first on Friday, Psalm 97 the first on Saturday. The cycle for vespers commenced on Sunday with Psalm 109 and continued throughout the week with the remaining Psalms (some Psalms were set aside for other hours). Other divisions of the Psalms are occasionally found, such as the Irish division of the three fifties (beginning at Psalms 1, 51, and 101). Such divisions would often be given prominence within the decorative program. Depictions of King David, author of many of the Psalms, frequently introduce the psalter (especially as historiated Beatus initials to Psalm 1), and prefatory cycles were often added, along with an illuminated calendar. Byzantine psalter illustration exerted an important influence on the West.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).