The Doobie Brothers' "Long Train Runnin'" (1973) is a rock track driven by one of the most distinctive guitar riffs of the era — a chugging, funky rhythm guitar pattern that owes more to Sly Stone than to the Allman Brothers. But beneath that riff, drummer Michael Hossack lays down a groove that's funkier than most dedicated funk records, with a loose, driving feel that caught the ear of hip-hop producers looking beyond the obvious sample sources.

The break's rock-funk hybrid quality gave producers a sound that was both familiar and unexpected — recognizable enough to trigger associations, but funky enough to work in a hip-hop context. Like the best rock samples in hip-hop, "Long Train Runnin'" demonstrated that groove transcends genre labels.