ZZ Top sing about the powerful Mexican radio stations that exposed the Texas trio to an influential blend of rock, country and blues music in the lyrics for 'Heard it on the X,' from their 1975 album 'Fandango!'

The "X" refers to the opening call letter for radio stations based south of the border, whose "Border Blaster" broadcasts reached deep into the United States because they weren't held to the same wattage limits as their American contemporaries. "Oh, from coast to coast and line to line / In every county there / I'm talkin' about that outlaw X / That was cutting through the air."

These stations also played a much wider and freer range of music, which the still-developing band -- Billy Gibbons, Dusty Hill and Frank Beard -- soaked up gratefully.  Or, in their words, "Country Jesus / Hillbilly blues / That's where I learned my licks."

The lyrics send a particular tribute to Dr. John R. Brinkley, an American doctor who launched XER-AM in the '30s as a means to promote his male enhancement operations -- which reportedly involved transplanted goat glands -- all across America until he was shut down in 1939.

The station also featured large chunks of music, and paved the way for others in the decades to follow, including XERF-AM, which helped launch the career of legendary DJ Wolfman Jack. Hence: "We can all thank Doctor B / Who stepped across the line / With lots of watts he took control / The first one of it's kind."

On 'Fandango''s version, Hill and Gibbons share the vocals, finishing each other's thoughts like a rock 'n' roll version of the Beastie Boys. Nowadays, Dusty typically sings 'Heard it on the X' solo in concert, usually after he and Billy silently drive the crowd into a frenzy by crossing their hands across their chests in an "X" pattern.

Final instructions from ZZ Top? "So listen to your radio / Most each and every night / Cause if you don't, you surely won't get the feeling right." To which we'd add, be sure to read our exclusive interview with Billy Gibbons, where he talks about the band's plans for their next studio album.

Watch ZZ Top perform 'Heard It on the X' from the 'Double Down' DVD