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I think we identify the decade of music by the chord progressions that seemed to dominate the popular music of the time. Certain eras favored different forumlas. Mostly music gets recycled and it comes around again to produce a sound that is current, even if we have "heard it before". If you take the music in the decade it was made, are there are some styles or songs that are more timeless than others? Maybe simple music ages best.
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