One of These Nights, the Eagles' fourth album, racked up a lot of firsts for the increasingly popular band. It's their first LP to reach No. 1. "Take It to the Limit" was their first gold-selling single. And it's the first (and only) time that guitarist Don Felder sang lead on an Eagles song ("Visions").

It's also the band's last album with co-founding guitarist Bernie Leadon, who left the group after the tour in support of One of These Nights. But that small bump in no way comes close to the list of achievements that mark the Eagles' fourth album. Before One of These Nights, they were a country-rock group that occasionally stretched its limits to middling results; after One of These Nights, the Eagles were one of the biggest rock bands in the world.

Six months before the June 1975 release of One of These Nights, the Eagles scored the first of their five chart-topping singles, "Best of My Love," from their 1974 album On the Border. During that time, and in the months leading up to its debut, the band – Felder, Leadon, guitarist Glenn Frey, drummer Don Henley and bassist Randy Meisner – recorded tracks for their fourth album in Miami and in Los Angeles.

The songs they came up with were more substantial than anything since their self-titled 1972 debut. And it was way more adventurous. From the monster disco-bottomed opener "One of These Nights" (featuring Henley's best vocal up to that point) to "Lyin' Eyes" (a six-minute country-jukebox tune sung by Frey and showcasing the group's stellar harmonies) to "Take It to the Limit" (with Meisner at his all-time best), the album signaled both the beginning and the end of Eagles eras.

Listen to the Eagles Perform 'One of These Nights'

Leadon – who came from the Flying Burrito Brothers and played banjo, mandolin and pedal steel in addition to guitar – helped define the Eagles' sound early on. He gave them their twang and roots. But as the group headed into more worldly directions, and as Frey and Henley pretty much assumed leadership, he was beginning to be left out musically more and more.

By the time the Eagles' next album, the world-conquering Hotel California, rolled around, Leadon was gone, as was most of the band's country leanings. One of These Nights was the firing shot toward that route. There were still traces – "Lyin' Eyes," the excellent "After the Thrill Is Gone" – but the overall takeaway here is that the Eagles were ready for the next level.

They reached it and more. The album stayed at No. 1 for five weeks, selling more than four million copies over the years and spawning three Top 5 singles: "One of These Nights," which made it to No. 1, "Lyin' Eyes" (No. 2) and "Take It to the Limit" (No. 4). It was also nominated for an Album of the Year Grammy, with "Lyin' Eyes" picking up a Record of the Year nod.

But it was all mere setup for Hotel California and, eventually, the band's legacy. Aside from the singles and maybe a couple of others, One of These Nights dips in consistency along the way. By the time they started working on Hotel California the following year, they had picked up Joe Walsh and figured out how to make a great album from start to finish. Would they have gotten there without One of These Nights? Possibly. But they probably wouldn't have taken the chance.

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