Beethoven is known as a bridge composer to the romantic era because

Beethoven was known as a bridge composer to the Romantic Era because of the way in which he pushed the boundaries to the extreme melodically and harmonically of the rules of the Classical Era. Late Beethoven, you could say, was one step away, from some pieces, being atonal. This is particularly seen in some of his later quartets and quintets.

-E :)

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