Of course, the unison ^5 forces attention away from the alto (lower right-hand voice) to the soprano, and it also (that is, like the unison ^3 from yesterday's post) aligns itself very cleanly with the formal design. The second line in the graph shows a simple harmonic transformation with one harmony for each section: first strain, contrasting middle, and reprise. In Riemannian terms, this is (L-followed-by-P) followed by the inverse (or, P-followed-by-L). L turns A major into C# minor, and P makes the latter C# major.
The third line fills in a few details for the first strain. Note that the symmetries in the harmonic patterning extend to the outer melodic voices, with the two neighbor-note figures. The inset takes this a little further by understanding the "essential" chromaticism of getting-into and getting-out-of the contrasting middle symmetrically, as well: E5 breaks up to E#5 at the beginning, but G-natural slumps down to F#5 at the end.
