Here's why:
* Thematic unity: This refers to the use of recurring themes, motives, or musical ideas throughout different movements of a work. It creates a sense of coherence and connection, even though the movements may be in different keys, tempos, or moods.
* Cyclic form: This is a specific type of musical form where a single theme or idea serves as a unifying element throughout the piece. It often involves the transformation or development of the theme in different movements, but the core idea remains present.
Examples of this can be seen in works by:
* Franz Schubert: His Symphony No. 9 in C Major ("The Great") is famous for its cyclical structure, with the main theme appearing in different guises throughout the work.
* Johannes Brahms: His String Quartet No. 1 in C Minor is another excellent example, where the first movement's main theme recurs in later movements, sometimes in altered forms.
Both thematic unity and cyclic form contribute to a sense of overall unity and purpose in a multi-movement work, even when the individual movements seem disparate at first glance.