DontRunMeOver

Well, R'n'B isn't explicity called rhythm and blues.

It's called R'n'B and I expect the change in the standard naming came partly from the same reasoning you just mentioned. Regardless, names don't have to accurately describe or sum up what they apply to. They can just be names.

Rock music doesn't usually involve rocks (unless the band have been enjoying some 'chinese rocks'). Metal music does, agreed, use things made with metal. But they are less metallic than the instruments used for jazz... and punk musicians tend to carry more metal upon their person than those in metal bands.

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