The psalter (Book of Psalms), which had always been a part of Jewish worship and personal prayer, was one of the key books of the Bible for Christian clergy, monks and lay people in the middle ages. It was used as a hymnal in church services and in the daily prayer of the monks. Parts of the mass that are sung are often based upon psalm verses. It was read as a personal prayerbook, many of the psalms probably recited from memory. Theologians studied and commented upon it. Most medieval psalters have calendars, to help the user organise daily prayer by reminding them which saint's day it was, and special prayers of appeal to God and the saints (litany). A psalter's original place of use can sometimes be deduced from the saints named in the calendar and litany because a monastery, church or town will have its favourites. The names in this psalter point to the Benedictine abbey at Hyde. The first letter B of Psalm 1 ("Beatus vir", "Blessed is the man") is nearly always the largest and most beautifully decorated in a medieval psalter. Here late medieval vine ornament fills its interior and forms a border around the first verses which cover the page. Medieval Christian decoration of psalters nearly always emphasises the psalms as prophecy of Jesus or explains them in terms of the Church. Here the vines could be seen as a sign of Jesus, who said "I am the vine".