Miserere mei, Deus, is Latin for "Have mercy on me, O God". It is a setting of Psalm 51 by Italian composer Gregorio Allegri. It was composed during the reign of Pope Urban VIII, probably during the 1630s, for the exclusive use of the Sistine Chapel during the Tenebrae services of Holy Week, and its mystique was increased by unwritten performance traditions and ornamentation. It is written for two choirs, of five and four voices respectively, singing alternately and joining to sing the ending in 9-part polyphony.
The version below is sung majestically by the choir of King's College, Cambridge. A haunting melody that helps us enter the spirit of holy week. Arguably one the most beautiful pieces of music ever written.