The president of the choir chose a particular psalm at his own will.
Some psalms, such as xxi, seem specially appropriate to the Passion.
In the Breviary, however, the Psalter is divided according to a special plan.
In the earliest period the use of the Book of Psalms in the Office was doubtless exactly similar to that which prevailed amongst the Jews.
The series, from Psalm 109 to Psalm 147 inclusively, are used at Vespers, five each day, except Psalms 117, 118 and 142, reserved for other hours.