The romance genre is by nature lyrical, and the two themes in my Romance for Oboe and Piano both fit in that general category. They contrast, however, in meter, tempo (the second theme is a little faster), and melodic contour. The first theme seems to me rather mournful, probably because of references to Lydian mode, while the second seems more cheerful, perhaps even amorous.
But the most obvious contrast is that the second theme is first presented in D major, which (like many a Romantic harmony) is remote on the circle of fifths from the home key of E-flat. So what is the harmonic function of D major harmony in the key of E-flat major? The root of the D major chord is the leading tone in E-flat major, and in general a leading tone strongly conveys dominant function. In fact, the D major harmony in this Romance progresses eventually through D minor to B-flat major, which is the conventional dominant harmony within the key of the piece. In the closing section, both themes appear in the home key of E-flat major, thus resolving the polarity between tonic and dominant.
The posing of tonic/dominant polarity and its subsequent resolution is the central harmonic idea of classical sonata form. Thus the Romance for Oboe and Piano can be viewed as a Romantic variant on the same idea.
Rachmaninoff’s Variations on a Theme of Corelli also use the leading tone as a secondary key center. This large-scale solo piano work, dating from 1931, in some ways resembles the composer’s better-known Rhapsody on a Theme of Paganini, which followed three years later. The 1931 work consists of a theme, twenty variations, and a coda. The overall tonality is D minor, but Variations XIV-XV use the remote secondary key center of D-flat major. The tonal motion from D minor to D-flat is accomplished in an Intermezzo inserted before Variation XIV. This middleground Schenker graph of the whole variation set comes from my PhD dissertation:
Note that D-flat is enharmonically equivalent to C-sharp, the leading tone in D minor. Since the leading tone is the strongest indicator of dominant function, the D-flat secondary key can be thought of as a dominant substitute within the variation set as a whole.
Since the D-flat major triad contains three tones, each of which can potentially function as a leading tone, there are three key centers in which it can potentially carry dominant function: D, G-flat, and A. In the key of G-flat major, of course, the D-flat triad is the traditional dominant; in the other two key centers, it can function as a dominant substitute.
The reference to D-flat major in the Corelli variation set presages the Rhapsody on a Theme of Paganini. The Rhapsody is set in A minor, like the Paganini Caprice No. 24 on which it is based, but the most famous of its variations, Variation XVIII, is set in D-flat major — which can again be regarded as substituting for dominant within the work’s overall harmonic motion.