Mozart composed his choral work, Ave verum corpus, K. 618, in 1791 for Anton Stoll, a friend of his and Joseph Haydn’s.  It is one of Mozart’s final compositions.  It is in the key of D major and, therefore, in a 2-sharp system.

The text is a fourteenth century Eucharistic hymn, a tribute to and celebration of  the sanctity of the body of Jesus who suffered and was sacrificed on the cross for mankind.  Although only 46 measures long, it contains a beautiful application of the missing pitch while the entire piece, in a single gesture, unfolds (almost) the entire PCA.  Both the missing pitch, spelled as F-natural, and PCA’s pc 3 converge simultaneously on a single noteworthy word of text.

After a two-measure introduction, the opening eight-measure phrase secures tonic harmony and moves to a bifocal cadence at m. 10, a half-cadence on the dominant that becomes the local tonic of the next phrase.  The only chromaticism so far is the G-sharp in m. 4, coming directly from A and immediately descending to G-natural in a lamenting gesture, true to the nature of the Communion text; it decorates the word Ave (“Hail,” of “Hail true body”).

Of course, G-sharp is the most obvious chromatic in the second phrase, but certainly not the most interesting.  The E-sharp in m. 14, an accented chromatic passing tone emphasized by Mozart, harmonizes the word immolatum, i.e., “sacrificed,” and stands out not only because of its strong rhythmic position in the measure, but also since it is the missing pitch.  The system, however, has not modulated due to the simultaneous sounding of the tonic, D, which would serve to keep us in the original 2 sharp system. It should  also be noted that at this point, in the absence of a D-sharp or E-flat, the PCA has yet to move further than pc 0.

The little four-measure phrase that is elided to the second phrase underscores the first period of music and allows the chorus to breathe, providing a rhythmic upbeat to the next phrase as well.  But this third phrase also introduces a D-sharp in m. 19, pc 1, part of the tasto solo, marking the beginning of the rising PCA and picking up pc 0 from the introduction.  Pc 1 resolves immediately to pc 2 at the end of the measure.

Things begin to move now with the presence of chromaticism in the next phrase, from m. 22 to 29, allowing a brief modulation to F major and initiating a short prolongation of a local  expansion in D minor, the parallel minor.  As we search for pc 3, we see it arrive in m. 25; the root of the chord is F-natural, the missing pitch, and it occurs—rather significantly—on the word perforatum (meaning “pierced”).  The systems are thrust downward to a 1-flat gamut as the text continues with the description of flowing water and blood moving toward the dominant of D minor.  The ever-present G-sharp, now in m. 28, could have resurrected the tonic 2-sharp system by furnishing the missing pitch of the 1-flat system.  However, the presence of F-natural in soprano parts of m. 28 makes that impossible.  As G-sharp and F-natural battle for hegemony, m. 29 returns D minor to D major; however, we will remain in a 1-flat system until G-sharp is systemically uncontested.  That will not happen until the final phrase.  Note that there is a conflict, then, between the key and the system … Mozart has not yet finished his task.

The fifth phrase, beginning in m. 30, provides several devices that allow this short piece to conclude successfully.  First, the PCA is completed:  Pc 4 turns the local D minor’s F-natural back to F-sharp and pc 5, the G in the bass of m. 31, G-sharp in m. 35 becomes pc 6, and its immediate resolution to A becomes pc 7.  Pc 8 is the B-flat in the bass of m. 39 and B-natural, pc 9, occurs in the upper voice parts of the next measure.  Pc 11, C-sharp, occurs in the concluding phrase’s structural dominant, m. 42, where it resolves in m. 43 to the tonic D, the regained pc 0.  Notice that pc 10, C-natural, although prominent in the measures just before where we would have expected it to have been presented, is not in a position where it can be expressed as a member of the PCA.  This is very common, and both pc 10, before a tonic cadence, and pc 6, before a significant dominant cadence (such as during the motion to the dominant of the dominant just before the second harmonic area in sonata form), are sometimes absent from the PCA since the drive to the cadence is so strong that it leaves the composer no time to insert the absent pitch without interfering with the rhythmic precision of the phrase.

Second, with pc 6, the G-sharp in m. 35, the 2-sharp system is finally regained as the text presents and repeats in mortis examine, “in the trial of death.”  With this presentation of G-sharp, there is no F-natural to prevent the systems from shifting back to 2 sharps.  Curiously, Mozart did not employ the final celebration of Jesus at the end of the hymn as stated in the original text; however, one could argue that, with D major and the 2-sharp system both regained, he allows the music to say “amen” for him in the final four measures.