Originally called As Fair As Morn, this piece is a simple example of three-part imitative counterpoint. The lowest part was originally written in the tenor clef and was intended for the tenor voice. The text is one of the earliest of the Ambrosian hymns, 6th century or earlier, this hymn is used from Easter Sunday until Ascension. In the Breviary revision of 1632 by Pope Urban VIII the hymn was so greatly altered that only three lines of the original remained and thus is really a different hymn entirely. The revised hymn can be found under the title of Ad regias Agni dapes.