Wolfgang Amadeus Mozart died on December 5th, 1791, halfway through writing his Requiem Mass in D minor. He had finished the opening Introit. He had drafted the Kyrie. He had sketched portions of several other movements down to the bass line and a few melodic fragments. Several movements existed only as ideas in his head — discussed with collaborators but never written down.

The Requiem had been commissioned anonymously by Count Franz von Walsegg, who wanted to claim the work as his own composition in memory of his deceased wife. When Mozart died with the commission half completed, his widow, Constanze Mozart, faced a difficult situation. She needed the commission money to support her two surviving children. The Count expected a finished Requiem.

She turned to Franz Xaver Süssmayr, a 25-year-old former student of Mozart's.

Süssmayr completed the Requiem from Mozart's surviving sketches, from his own memory of conversations with Mozart, and from pure stylistic imitation. He wrote three movements substantially or entirely from scratch — the Sanctus, the Benedictus, and the Agnus Dei. He extended several others where Mozart had left only fragments.

Constanze had Süssmayr forge Mozart's signature on the completed manuscript and submitted the work to Count Walsegg as a finished Mozart composition. The Count paid in full. The forgery was not publicly acknowledged until 1825, more than three decades later.

Today, what audiences hear as Mozart's Requiem is roughly half Mozart and half Süssmayr. Modern scholarly editions distinguish the two clearly. Several alternative completions have been written — by Robert Levin, Richard Maunder, and others — based on stricter readings of Mozart's surviving sketches.

But most performances still use Süssmayr's version. It is the version that has been performed for over two hundred years.