Those of you that are in the habit of saying Prime as part of your daily devotions will have noticed that whenever Lauds 2 is said, Prime has four instead of three Psalms.After the first three Psalms have been completed you will find the Rubric:
When Psalm 51, Miserere mei, is said at Lauds, here is added:
Which is to say that whenever Lauds is said of Lauds 2 – which is the only time when Lauds always begins with Psalm 51 – a fourth Psalm must be added to the Office of Prime. The Psalm which must be added follows immediately upon the Rubrical note and after it the Antiphon is said as usual and the Office resumes as usual. This is yet another way in which the Office changes during Lent. The changes in the Office reflect a more penitential “mood” for the Office which set it apart from other Season and the ways in which the Office is said.