(Photo by graphicstock.com)

In a courtroom in Los Angeles, a U.S. District Judge has been hearing testimony on whether Jimmy Page and Robert Plant of the legendary rock band Led Zeppelin borrowed or stole a musical phrase as part of creating their signature song "Stairway to Heaven." Hear the two pieces of music in this NPR story.

What makes them sound alike?

In many cases, music's shared history. Much modern music is derived from the blues or other elements that have existed for years.

“It’s almost impossible to write a song, especially these days that doesn’t have an element from some other song,” said musician Adam Roberts, who teaches at Glendale Community College.