The eccentric count Franz von Walsegg commissioned the Requiem from Mozart anonymously through intermediaries.
The count, an amateur chamber musician who routinely commissioned works by composers and passed them off as his own, wanted a Requiem Mass he could claim he composed to memorialize the recent passing of his wife.
Mozart received only half of the payment in advance, so upon his death his widow Constanze was keen to have the work completed secretly by someone else, submit it to the count as having been completed by Mozart and collect the final payment.
Joseph von Eybler was one of the first composers to be asked to complete the score, and had worked on the movements from the Dies irae up until the Lacrymosa.
In addition, a striking similarity between the openings of the Domine Jesu Christe movements in the requiems of the two composers suggests that Eybler at least looked at later sections.
After this work, he felt unable to complete the remainder and gave the manuscript back to Constanze Mozart.

The task was then given to another composer, Franz Xaver Süssmayr.
Süssmayr borrowed some of Eybler's work in making his completion, and added his own orchestration to the movements from the Kyrie onward, completed the Lacrymosa, and added several new movements which a Requiem would normally comprise: Sanctus, Benedictus, and Agnus Dei.
He then added a final section, Lux aeterna by adapting the opening two movements which Mozart had written to the different words which finish the Requiem mass, which according to both Süssmayr and Mozart's wife was done according to Mozart's directions.
Some people consider it unlikely, however, that Mozart would have repeated the opening two sections if he had survived to finish the work.

Other composers may have helped Süssmayr.
The Agnus Dei is suspected by some scholars to have been based on instruction or sketches from Mozart because of its similarity to a section from the Gloria of a previous mass (Sparrow Mass, K. 220) by Mozart, as was first pointed out by Richard Maunder.
Others have pointed out that at the beginning of the Agnus Dei, the choral bass quotes the main theme from the Introitus.
Many of the arguments dealing with this matter, though, center on the perception that if part of the work is high quality, it must have been written by Mozart (or from sketches), and if part of the work contains errors and faults, it must have been all Süssmayr's doing.

Another controversy is the suggestion (originating from a letter written by Constanze) that Mozart left explicit instructions for the completion of the Requiem on "a few scraps of paper with music on them... found on Mozart's desk after his death."
The extent to which Süssmayr's work may have been influenced by these "scraps" if they existed at all remains a subject of speculation amongst musicologists to this day.

The completed score, initially by Mozart but largely finished by Süssmayr, was then dispatched to Count Walsegg complete with a counterfeited signature of Mozart and dated 1792.
The various complete and incomplete manuscripts eventually turned up in the 19th century, but many of the figures involved left ambiguous statements on record as to how they were involved in the affair.
Despite the controversy over how much of the music is actually Mozart's, the commonly performed Süssmayr version has become widely accepted by the public.
This acceptance is quite strong, even when alternative completions provide logical and compelling solutions for the work.
The eccentric count Franz von Walsegg commissioned the Requiem from Mozart anonymously through intermediaries. The count, an amateur chamber musician who routinely commissioned works by composers and passed them off as his own, wanted a Requiem Mass he could claim he composed to memorialize the recent passing of his wife. Mozart received only half of the payment in advance, so upon his death his widow Constanze was keen to have the work completed secretly by someone else, submit it to the count as having been completed by Mozart and collect the final payment. Joseph von Eybler was one of the first composers to be asked to complete the score, and had worked on the movements from the Dies irae up until the Lacrymosa. In addition, a striking similarity between the openings of the Domine Jesu Christe movements in the requiems of the two composers suggests that Eybler at least looked at later sections. After this work, he felt unable to complete the remainder and gave the manuscript back to Constanze Mozart.
The task was then given to another composer, Franz Xaver Süssmayr. Süssmayr borrowed some of Eybler's work in making his completion, and added his own orchestration to the movements from the Kyrie onward, completed the Lacrymosa, and added several new movements which a Requiem would normally comprise: Sanctus, Benedictus, and Agnus Dei. He then added a final section, Lux aeterna by adapting the opening two movements which Mozart had written to the different words which finish the Requiem mass, which according to both Süssmayr and Mozart's wife was done according to Mozart's directions. Some people consider it unlikely, however, that Mozart would have repeated the opening two sections if he had survived to finish the work.
Other composers may have helped Süssmayr. The Agnus Dei is suspected by some scholars to have been based on instruction or sketches from Mozart because of its similarity to a section from the Gloria of a previous mass (Sparrow Mass, K. 220) by Mozart, as was first pointed out by Richard Maunder. Others have pointed out that at the beginning of the Agnus Dei, the choral bass quotes the main theme from the Introitus. Many of the arguments dealing with this matter, though, center on the perception that if part of the work is high quality, it must have been written by Mozart (or from sketches), and if part of the work contains errors and faults, it must have been all Süssmayr's doing.
Another controversy is the suggestion (originating from a letter written by Constanze) that Mozart left explicit instructions for the completion of the Requiem on "a few scraps of paper with music on them... found on Mozart's desk after his death." The extent to which Süssmayr's work may have been influenced by these "scraps" if they existed at all remains a subject of speculation amongst musicologists to this day.
The completed score, initially by Mozart but largely finished by Süssmayr, was then dispatched to Count Walsegg complete with a counterfeited signature of Mozart and dated 1792. The various complete and incomplete manuscripts eventually turned up in the 19th century, but many of the figures involved left ambiguous statements on record as to how they were involved in the affair. Despite the controversy over how much of the music is actually Mozart's, the commonly performed Süssmayr version has become widely accepted by the public. This acceptance is quite strong, even when alternative completions provide logical and compelling solutions for the work.
Requiem (Mozart)