Final answer:
The main difference between Classical and Romantic music came from attitudes towards the rules. Composers in the eighteenth century were focused on structure, melody, and harmony, while composers in the Romantic era challenged traditional norms and experimented with atonality, unexpected notes, and altered scales. Some of these modernist compositions became classics while others are more historical in nature.
Step-by-step explanation:
The main difference between Classical and Romantic music came from attitudes towards these "rules”. In the eighteenth century, composers were primarily interested in forms, melodies, and harmonies that provided an easily-audible structure for the music.
In contrast, the Austrian composer Arnold Schoenberg (1874 - 1951) invented a form of orchestral music that remains more of an important influence to avant-garde musicians and composers than something actively listened to by mainstream audience. Schoenberg's major innovations consisted of experiments with atonality - music without a central, binding key - and a newly-invented twelve-tone scale of his own creation. Schoenberg was among the first to defy the entire tradition of western music in his experiments. Ever since the Renaissance, western musicians had worked in basically the same set of scales. As a result, listeners were "trained" from birth to expect certain sounds and certain rhythms in music. Schoenberg deliberately subverted those expectations, inserting dissonance and unexpected notes in many of his works.
Modernism was not confined to literature and the visual arts, however. Some composers and musicians in the first decades of the twentieth century sought to shatter musical traditions, defying the expectations of their listeners by altering the very scales, notes, and tempos that western audiences were used to hearing. Some of the resulting pieces eventually became classics in their own right, while others tended to become part of the history of music more so than music very many people actually listened to.