While many an iconic song has been written about blossoming love (Etta James' 'At Last,' Al Green's 'Let's Stay Together') or romantic strife (Nina Simone's 'Don't Let Me Be Misunderstood,' Teddy Pendergrass' 'Love T.K.O.'), plenty more R&B records carry urgent social and political messages. Some of the most fruitful periods in modern music came in the world of R&B, including Motown, the Philly soul scene, and even the 21st century wave of alternative R&B.

Like jazz before it, and hip-hop in the ensuing years, R&B came out of Black America, bringing previously marginalized stories and perspectives into the ears of millions of listeners, first in the U.S. Whether the instrumental is a lush Motown arrangement with swooning horns and strings or a slinky digital creation, it has to add depth to the record and make the singer sound better. A powerful, vulnerable vocal performance-the kind that artists like Marvin Gaye, Gladys Knight, and Solange have delivered in spades-is key. The sound of R&B has changed tremendously in the last 75 years, but the ingredients that make a timeless soul track remain the same.