How Gregg Allman turned late-night despair into the classic Whipping Post. “I didn’t know exactly what I was doing. I just did it.”

In 1969, Gregg Allman woke in the middle of the night at a friend's house in Jacksonville, Florida, with the music and lyrics for what would become Whipping Post already forming in his head. Unable to find paper or a pen, and needing to stay quiet so as not to disturb bandmate Berry Oakley, his…