Metallica's “Sad but True” was lowered into D standard tuning during recording.
The song was initially written and demoed in E standard. Producer Bob Rock noticed that the album's songs were all in the key of E and suggested tuning down, drawing on the lower tuning used on Mötley Crüe's Dr. Feelgood, an album Metallica admired.
The band tuned the instruments down to D, and the main riff gained the unusually massive weight associated with the finished recording. D standard means every string is lowered by one whole step from ordinary E standard tuning.
“Sad but True” was released as the fifth and final commercial single from the album in February 1993. The song's early idea reportedly dates to the Master of Puppets era, several years before the Black Album sessions.