The Eagles launched Glenn Frey to fame, but the singer-songwriter and multi-instrumentalist was already a veteran of the recording studio before he ever helped found the band. Prior to the Eagles, Frey was half of a duo called Longbranch Pennywhistle that released one album — and his duo partner would also go on to play an important role in the career of the Eagles.

Born in Detroit, Frey made his way to California to launch his career, where he met a singer-songwriter from Texas named John David Souther. They formed a duo, and their blend of early country-rock and folk with vocal harmonies attracted a lot of attention when they began playing at the famed Troubadour in Los Angeles, which led to a recording contracting with Amos Records and a self-titled debut album in 1969.

Legendary record producer Jimmy Bowen was a technical supervisor for Longbranch Pennywhistle's lone album, which featured appearances from top-flight session players including James Burton, Buddy Emmons, Larry Knechtel, Joe Osborn, Jim Gordon, Ry Cooder and Doug Kershaw.

Frey and Souther wrote all of the songs, both individually and collectively, resulting in an album that AllMusic.com described as "pleasant yet unthrilling." The tight, focused songwriting that both men would later employ is still in its formative stages on many of the songs, as you can hear by listening to two selections from the album below.

The Longbranch Pennywhistle album was unsuccessful, and the duo disbanded in 1970. The following year, Linda Ronstadt drafted Frey to play in her new backing group, which she envisioned as a group of country-rock all-stars. Frey first met Don Henley, Bernie Leadon and Randy Meisner on that gig; Henley had previously played drums in a group called Shiloh, which Kenny Rogers first discovered in Dallas and brought to Los Angeles to record, Meisner had played in Rick Nelson's Stone Canyon Band, and Leadon had performed with the Flying Burrito Brothers.

The chemistry between the band members was so strong that they split off to form the Eagles, releasing their self-titled debut album in 1972, which catapulted them to instant success. They would go on to become one of the biggest superstar acts of the decade, and Souther was a frequent songwriting collaborator, co-writing Eagles hits including "The Best of My Love," "Heartache Tonight," "James Dean," "Victim of Love" and "New Kid in Town."

Souther also went on to a successful solo career, scoring hits including "You're Only Lonely" and "Her Town Too," a duet with James Taylor. Linda Ronstadt, Dan Fogelberg, Bonnie Raitt and more have also recorded his songs, and he was inducted into the Songwriters Hall of Fame in 2013.

See Inside Glenn Frey's Sprawling California Mansion:

See Inside Don Henley's Hollywood Bungalow: