When and why did rap music shift from being exclusively an underground or street form to a commercial commodity for mass consumption?

In 1979, Sugarhill Gang a notorious rap group that was produced by Sylvia Robinson released a Hip-Hop track called "Rappers Delight".

Besides appealing to a wider audience the single track recorded over 2 million copies in sales within the United States which earned Sugar Hill Records approximately US$3.5 million.

This realization of how much money could be made from rap became the tipping point for rap which moved from being street music to a commercial commodity.

Since then, rap has become sophisticatedly commercial with lots of rappers who have gone on to sell millions of albums in a week. Eminem is currently the highest-selling solo rapper of all time.

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we could tell by the rotten smell, that something putrid was in our trash can.