Byrd's five-part setting of the Compline hymn 'Christe qui lux es et dies' (Christ, who art the light and day) is unique amongst his compositions. Its only source is the beautiful and useful collection of Tudor musical manuscripts copied by Robert Dow in 1580 into his celebrated Dow's part-books. Dow cross-referenced Byrd's setting to those of Robert White and it's possible that Byrd based this setting on White's work.
White's setting was intended for liturgical performance and has the odd-numbered verses set to plainchant. Byrd however did not completely follow White's example. Instead he uses the chant only for the beginning and end verses setting the remaining five verses (vv two to six). He starts the cantus firmus in the bass in verse two moving up through the voices in each successive verse by fifths and fourths. He ends with a short poplyphonic Amen.
You can hear it sung below by The Cardinall's Musick conducted by Anderw Carwood. Enjoy :-).
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