Romantic music has its roots in the Classical music style. The development of forms and harmonic ideas that became prominent in the Classical period expanded in the Romantic period. This gave rise to both similarities and differences between the two periods of music. As composers were moving toward defining the individual as opposed to the group perspective of music, Romantic composers began to stretch boundaries of form and harmonic function.
One of the most enigmatic composers of both periods is Beethoven. His music contradicts itself in terms of style. His early symphonies sound like Classical period pieces, specifically his first two symphonies. From there, the music changes into a Romantic style. The strict forms of Classical music started to disintegrate as he wrote the highly programmatic "Fifth Symphony." This symphony made use of a 4-note victory motive in which the music depicts a tale of victory in war. This contrasts with the absolute Classical period that dealt in musical themes and not necessarily human expression.
Classical composers were trying to get away from the comparably chaotic music of the Baroque period. However, Romantic composers did not try to move away from the music of the Classical period. This distinction in philosophies is important in comparing the difference between the two styles. While the Classical period sought to create something entirely new, the Romantic period was content to expand and develop the ideas of the Classical period. This overlapping of ideas is why many early Romantic composers may be hard to distinguish from late Classical composers.
The Classical periods was highly intent on preserving order and presenting melodies in the clearest way possible. Because of this, chords in the Classical period were very straightforward and based heavily on the major-minor scale relationships. This attitude toward musical rules changed in the Romantic period. Composers in the Romantic period began expanding sonata structure, obscuring the melody with more advanced and chromatic chords, and creating a new style of music that expressed the dramatic and not necessarily the physical aspects of music. The Romantic generation cast aside ideas that did not serve their immediate needs and kept concepts that enhanced their music.
Classical composers were content to stay within a certain boundary of what constituted acceptable music. Chord resolutions were always the same, the relationship between movements, sections and keys maintained proportion. The Romantic composers stretched these boundaries, introduced new chords, unusual key changes and in many ways went against the procedures and policies that were developed in the Classical period. While the forms such as sonata, symphony and even fugue remained the same, the interpretation of these forms changed drastically by greatly expanding the length and character of these forms.
The Classical period had a musical style that was consistent. If you were a composer in this time period, you knew what was expected of you. Haydn was a huge influence on the development of Classical music with Mozart serving to refine and perfect the style. Beethoven started writing in a Classical style only to abandon and lead the way toward a more Romantic style. The early Romantic composers such as Brahms and Schubert kept more closely with the Classical traditions and simpler chord structure. Meanwhile, during the latter part of the Romantic period, composers such as Wagner and Strauss were stretching the very essence of tonality. These composers paved the way for the next generation of composers.