Answer:
Although we might reasonably draw the boundary line between 19th century music and that of the 20th century at the beginning of World War I (1914), important developments took place well before that date. One of the most important technical developments that took place in late 19th century music was the gradual breakdown or dissolution of the system of tonality that had been an implicit aspect of Western European music since the beginning of the Baroque period. Tonal music continued to be written (and continues today); however, the practice has become a matter of choice rather than an unstated assumption.
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