When was classical music a popular music?

I think classical music was popular in the 1900's.

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Broadly defined, "Classical music" can refer to any art music from the 9th Century to the present. The distinctions are becoming blurred, but art music is music other than the popular forms listened to casually, or enjoyed by the majority of young people, like rap, jazz, all the forms or rock, country, etc. Classical music in this sense is being written today. People are writing large-scale works for symphony orchestras for example, that follow structural ideas and that often deal with sound that is free of lyrics. Many young people today think that lyrics and music are identical, or that the music is incidental or less important. No value judgments here-- just observations.

On the other hand, the "Classical Period" is more narrowly defined as being roughly between the periods known as Baroque and Romantic. This period is about 1730/1750 to 1815/1820. Music from this "Classical Period" is just a part of the "Classical music" as thought of in the broad definition. Mozart is the highlight of the Classical Period, and Beethoven is sometimes regarded as an important transition figure between Classical and Romantic periods.

So, Classical Period music was indeed popular music during the Classical Period, between the early 1700's and the early 1800's. But classical music is wildly popular even today, and it is not likely that the classical craze will end any time soon. See links for more.

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Classical music is still popular, but it seems that 'popular' depends on how many people enjoy it. Besides that, the music today was formed on the foundation of classical music, no matter how different it may sound. Classical music wasn't the beginning of music, but it has had a big impact on music today. So, you could easily say it's still popular today.