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The Principle of Thematic Unity

An important general principle developed by Romantic composers was that of thematic unity. There was an increasing tendency to maintain some of the same thematic material throughout whole works, even (or especially) when these works were in many movements.

In nineteenth-century symphonies and other such works, several different levels of thematic unity can be distinguished:

Most obviously, themes from one movement may come back literally and quite clearly in other movements. We have already heard this happen in Beethoven’s Fifth Symphony, when the scherzo theme returns in the last movement.
In other compositions, new versions of a single theme are used at important new points in the music, either later in the same movement or in later movements. While these new versions are really nothing more than variations of the original theme, this procedure differs fundamentally from Classical theme and variations form (see page 169). In Classical variation form, the theme is an entire tune, and the variations follow one another directly. In the new Romantic procedure, the theme is (generally) much more fragmentary than a tune, and the new versions of the theme appear at irregular intervals in the midst of other, unrelated music.
The term thematic transformation is used for this variation-like procedure in Romantic music, whereby short themes are freely varied at relatively wide and unpredictable intervals of time. A precedent for it can be traced to works such as Beethoven’s Fifth Symphony, where the motive of the first movement is evoked freely in each of the later ones.
In still other nineteenth-century pieces, we hear themes with even looser relationships among them. Clearly different, they nonetheless exhibit mysterious inner similarities — similarities that seem to help unify the music, though they are too shadowy to count as transformations in the Romantic definition, let alone as variations in the Classical style. Wagner’s operas are famous for such themes.

Of all the levels of thematic unity employed by nineteenth-century composers, this last is the most typical of all. Vague similarity rather than clear likeness, suggestion rather than outright statement, atmosphere rather than discourse, feeling rather than form: All these go to the heart of Romanticism. We cannot appreciate Romantic music fully if we approach it in too literal a frame of mind. In much of this music, the special spontaneous form of the individual piece, as distinct from standard forms such as sonata and rondo form, is tied to the principle of thematic unity. Listening to Romantic music requires ears that are not only attentive but also imaginative, exploratory, and more than a little fanciful.