The psalter was a specialized book containing only the 150 psalms, which were believed to have been written by King David. It is divided into the Divine Office, a routine used by ecclesiastics that involves the recitation and reading of religious texts at specified hours of the day. The canonical hours that divided the day into eight prayer hours are Matins (before dawn), Lauds (dawn), Prime (early morning), Terce (third hour, 9am), Sext (sixth hour, noon), None (ninth hour, 3pm), Vespers (sunset), and Compline (end of the day). The Psalter was used by the clergy for the liturgy and were also commissioned by private individuals for their personal devotion. For this reason, psalters are one of the most illuminated of all medieval books.
Sandler, Lucy Freeman. “Psalter.” Oxford Art Online. 2003, updated September 22, 2014.