When the gospel of the South met the rhythm & blues of the Midwest, it brought the spirituality of a Sunday Revival into the boogie-woogie world of rhythm & blues. Out of this union, Soul was born. The backbone of Soul is a solid rhythm section with a decidedly secular groove. The guitar usually takes a back seat to a Hammond B-3 organ, with a robust horn section in tow. Almost all Soul centers around impassioned, soaring vocals with occasional improvisational touches. Artists like Sam Cooke, Aretha Franklin and Al Green verge on blurring the line that separates the soul singer from the evangelical preacher. Detroit and Philadelphia were hotbeds of mainstream Soul as it crossed over to the masses in the mid-'60s, while developments in Alabama's Muscle Shoals and Memphis' famous Stax/Volt studios kept Soul music relevant through to the birth of Funk. Soul's effect on American popular music reverberates through the Blue-Eyed Soul of the '70s on to today's R&B hits.