The Rule of Saint Benedict: Chapter 12

Now, we move into the way Lauds is supposed to be conducted. Lauds is similar to Matins, or Orthros in the Eastern churches. Lauds is so called because, "the three last psalms of the psalter (148, 149, 150), the Laudate psalms, which in former versions of the Lauds of the Roman Rite occurred every day, and in all of which the word laudate is repeated frequently." (Wikipedia)

Chapter 12
The Celebration of the Solemnity of Lauds

Sunday Lauds begin with Psalm 66, said straight through without a refrain.

Then Psalm 50 follows with an "alleluia" refrain. 

Lauds continues with Psalms 117 and 62, 

The Canticle of the Three Young Men, Psalm 148 through 150, a reading of the Apocalypse recited by heart and followed by a responsory, an Ambrosian Hymn, a versicle, the Gospel Canticle, the litany, and the conclusion.


***NOTE***

You may have noticed that the phrase "Ambrosian Hymn" appears somewhat frequently while discussing these formal prayers. Saint Ambrose is styled as the Father of Church Song [in the West], and his style of hymns bears his name.

"The rule of St. Benedict employed the term; and Walafridus Strabo[5] notes that, while St. Benedict styled the hymns to be used in the canonical hours Ambrosianos, the term is to he[sic] understood as referring to hymns composed either by St. Ambrose or by others who followed his form;" (Wikipedia)

 I wanted to add the above information because I was unfamiliar with the term, though I figured it was much like what I stated above.