The beginning to Led Zeppelin's "Stairway to Heaven" is one of the most famous guitar melodies of all time.
But an instrumental piece called "Taurus," written by 60's rock act Spirit in 1968 is very similar - even too close to tell the difference.
Think it's just a coincidence? Spirit and Zeppelin crossed paths several times during the three years between the release of "Taurus" and "Stairway to Heaven."
Led Zeppelin's first American concert was in 1968 at the Denver Auditorium Arena - where they opened for Spirit.
Now all these years later, Spirit is finally lawyering up.
"It's been a long time coming," attorney Francis Alexander Malofiy said in a feature story for Business Week. Malofiy represents a songwriting trust for the late Randy California, guitarist for the band.
California, born Randy Wolfe and later nicknamed by Jimi Hendrix, had a tough life after Spirit broke up. There are reports of him bartering songs for money in the mid-90's.
Meanwhile, "Stairway" is still a goldmine. The total value of the song is an estimated $572 million.
It's not Led Zeppelin's first legal battle over plagiarism. They have settled four previous claims out of court, including a battle with singer/songwriter Jake Holmes over the hit "Dazed and Confused."
Holmes sued in 2010 and was given a songwriting credit.