“Black Dog,” named after a Labrador dog that wandered around the studio while Led Zeppelin was recording, sees Robert Plant lamenting about a failed relationship with a gold digger.

Although the title was inspired by an actual dog, the term “black dog” has been used to refer to negative emotions, as well as a Hellhound, a mythical creature that follows someone before they die.

The song’s iconic guitar riff was considered hard to play even by Jimmy Page. This may partially be because the riff (written by bassist John Paul Jones) is in the time signature of 9/8, while drummer John Bonham played in 4/4.