The autograph manuscript shows the finished and orchestrated movement of Introit in Mozart's hand, and detailed drafts of the Kyrie and the sequence of Dies irae as far as the first eight bars of the Lacrimosa, and the Offertory.

It cannot be shown to what extent Süssmayr may have depended on now lost "scraps of paper" for the remainder; he later claimed the Sanctus and Benedictus and the Agnus Dei as his own.

The following table shows for the eight sections in Süssmayr's completion with their subdivisions: the title, vocal parts (solo soprano (S), alto (A), tenor (T) and bass (B) [in bold] and four-part choir SATB), tempo, key, and meter.

The Requiem begins with a seven-measure instrumental introduction, in which the woodwinds (first bassoons, then basset horns) present the principal theme of the work in imitative counterpoint.

The courses of the melodies, whether held up or moving down, change and interlace amongst themselves, while passages in counterpoint and in unison (e.g., Et lux perpetua) alternate; all this creates the charm of this movement, which finishes with a half cadence on the dominant.

A final portion in a slower (Adagio) tempo ends on an "empty" fifth, a construction which had during the classical period become archaic, lending the piece an ancient air.

The Dies irae ("Day of Wrath") opens with a show of orchestral and choral might with tremolo strings, syncopated figures and repeated chords in the brass.

This material is repeated with harmonic development before the texture suddenly drops to a trembling unison figure with more tremolo strings evocatively painting the "Quantus tremor" text.

The final quarter notes of the bass soloist herald the arrival of the tenor, followed by the alto and soprano in dramatic fashion.

On the text Cum vix justus sit securus ("When only barely may the just one be secure"), there is a switch to a homophonic segment sung by the quartet at the same time, articulating, without accompaniment, the cum and vix on the "strong" (1st and 3rd), then on the "weak" (2nd and 4th) beats, with the violins and continuo responding each time; this "interruption" (which one may interpret as the interruption preceding the Last Judgment) is heard sotto voce, forte and then piano to bring the movement finally into a crescendo into a perfect cadence.

The chords begin piano on a rocking rhythm in 128, intercut with quarter rests, which will be reprised by the choir after two measures, on Lacrimosa dies illa ("This tearful day").

Then, after two measures, the sopranos begin a diatonic progression, in disjointed eighth-notes on the text resurget ("will be reborn"), then legato and chromatic on a powerful crescendo.

Between these thematic passages are forte phrases where the choir enters, often in unison and dotted rhythm, such as on Rex gloriae ("King of glory") or de ore leonis ("[Deliver them] from the mouth of the lion").

An overtaking chromatic melody on Fac eas, Domine, de morte transire ad vitam ("Make them, O Lord, cross over from death to life") finally carries the movement into the dominant of G minor, followed by a reprise of the Quam olim Abrahae promisisti et semini eius fugue.

The words "Quam olim da capo" are likely to have been the last Mozart wrote; this portion of the manuscript has been missing since it was stolen at 1958 World's Fair in Brussels by a person whose identity remains unknown.

The third phrase, (C), is a solemn ringing where the winds respond to the chords with a staggering harmony, as shown in a Mozartian cadence at mm.

Preces meae non sunt dignae, sed tu, bonus, fac benigne, ne perenni cremer igne.

Lord Jesus Christ, King of glory, Liberate the souls of the faithful departed from the pains of hell and from the bottomless pit.

Fac eas, Domine, de morte transire ad vitam, Quam olim Abrahae promisisti et semini ejus.

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.

Constanze had a difficult task: she had to keep secret the fact that the Requiem was unfinished at Mozart's death, so she could collect the final payment from the commission.

For a period of time, she also needed to keep secret that Süssmayr had anything to do with the composition of the Requiem, to allow Count Walsegg the impression that Mozart wrote the work entirely himself.

In 1798, Friedrich Rochlitz, a German biographical author and amateur composer, published a set of Mozart anecdotes that he claimed to have collected during his meeting with Constanze in 1796.

Also in 1798, Constanze is noted to have given another interview to Franz Xaver Niemetschek,[18] another biographer looking to publish a compendium of Mozart's life.

He published his biography in 1808, containing a number of claims about Mozart's receipt of the Requiem commission: This account, too, has fallen under scrutiny and criticism of its accuracy.

This work likely influenced the composition of Mozart's Requiem; the Kyrie is based on the "And with His stripes we are healed" chorus from Handel's Messiah, since the subject of the fugato is the same with only slight variations by adding ornaments on melismata.

The melody is used by many composers e.g. in Bach's cantata Meine Seel erhebt den Herren, BWV 10 but also in Michael Haydn's Requiem.

H. C. Robbins Landon argues that this Amen Fugue was not intended for the Requiem, rather that it "may have been for a separate unfinished mass in D minor" to which the Kyrie K. 341 also belonged.

In contrast, Carl Czerny wrote his piano transcription for two players, enabling him to retain the extent of the score, if sacrificing timbral character.

Franz Liszt's piano solo (c.1865) departs the most in terms of fidelity and character of the Requiem, through its inclusion of composition devices used to showcase pianistic technique.