Miserere mei, Deus (Allegri) (Wikipedia)

Allegri's Miserere's Wikipedia article is a community-edited encyclopedia entry, covering the nine-voice setting of Psalm 51, its reservation to exclusive performance in the Sistine Chapel under threat of excommunication for copying it, and the fourteen-year-old Mozart's 1770 memory transcription, later published by Charles Burney in 1771. Its strength is that these are the kind of stable, documentable composition and transmission history facts a general reference handles well. Its limit is that a claim about the piece's specific liturgical function within Holy Week Tenebrae services needs a source examining that directly, not this general overview.