Big Bill Broonzy was the bridge. Country blues to urban Chicago, acoustic to electric, American juke joint to European concert hall. He wrote Key to the Highway -- a song so sturdy that Eric Clapton, Derek and the Dominos, and a hundred others have covered it without improving on the original.

He was a fiddle player before he took up guitar, and he brought a melodic sophistication to the blues that nobody else had. He recorded hundreds of sides under dozens of names, backed just about every blues singer who passed through Chicago, and became the blues ambassador to Europe -- playing folk clubs and concert halls for audiences who'd never seen a Black American musician in person.

When the folk revival came calling, Broonzy adapted -- playing acoustic again, telling stories between songs, becoming an elder statesman to a generation of young white folksingers who'd never set foot in a juke joint. He was the most recorded bluesman of the 1930s and the most important bridge figure in blues history. Key to the Highway is the sound of a man who'd been everywhere and knew the way.

Big Bill Broonzy was the bridge between country blues and Chicago, between the juke joint and the concert hall. Key to the Highway has been covered by everyone and topped by no one. The blues ambassador before there was such a thing.