Stevie Wonder didn't have to prove anything during the last half of the '70s.

He'd just spent the first part of the decade on one of the greatest runs in popular music, with classic album after classic album: Music of My Mind and Talking Book (both 1972), Innervisions (1974), Fulfillingness' First Finale (1974). But Wonder didn't slow down, immediately beginning work on his 18th album, tentatively titled Let's See Life the Way It Is before it changed to Songs in the Key of Life for its Sept. 28, 1976, release.

By the time Wonder wrapped up recording a few months into 1976, he had enough songs for a double record plus a four-song EP to be included with the package. It was a monumental undertaking and effort by one of the most significant artists of the era, and its release only made him bigger and more vital.

Looking back at Songs in the Key of Life now, its influence and excellence have increased immeasurably over the years. It was a great album in 1976; decades removed from that initial blast, it's even more extraordinary. Name any classic pop or soul (or even rock) album of the '80s, '90s and '00s, and chances are you'll be able to trace its lineage to Wonder's eternal masterpiece.

Listen to Stevie Wonder's 'Sir Duke'

Like on his other records from the '70s, Wonder handled many of the instruments himself on Songs in the Key of Life, including keyboards, drums and harmonica. But he also brought in the biggest support crew he'd had since declaring his creative independence from Motown on 1971's Where I'm Coming From. Horn players, percussion players, guitar players and a small community of backing singers are woven throughout the project's 21 tracks. In all, more than 130 musicians are given some kind of credit on the album.

Recording was done across four studios: Crystal Sound in Hollywood, where the majority of Songs in the Key of Life was completed, and the Record Plants in Los Angeles and Sausalito and New York's the Hit Factory. Wonder worked nonstop during the long months, pushing his collaborators – even the famous ones like George Benson, Herbie Hancock and Minnie Riperton – to follow his lead.

They emerged with one of the century's greatest albums, a boundary-smashing piece that spotlights inner-city living, jazz history, religion, the Black experience, the female experience and boundless love for friends and family. (That's baby daughter Aisha Morris heard in the bathtub at the end of "Isn't She Lovely," which was written for her.) The music by turns is joyful, angry, sorrowful and romantic. Wonder pretty much nailed it when he called the album Songs in the Key of Life.

Listen to Stevie Wonder's 'I Wish'

From the opening "Love's in Need of Love Today," the music sounds fuller and more in control than anything Wonder had recorded before. He'd become more polished, and less spontaneous, as the years went on, and that move begins here. But unlike some of those later records that don't have room to breathe at times, these songs overflow with life and a room-filling sound that fits Wonder's last proper album in a decade where he became untouchable.

Some of Key's best songs became hit singles: "I Wish" and "Sir Duke" both reached No. 1; "Another Star" and "As" made it into the Top 40. But the album's riches can be heard throughout, as tracks cross from one side to the other: "Knocks Me Off My Feet" and "Joy Inside My Tears" are love songs, "Pastime Paradise" and "Black Man" are more political in nature. The rest of the record falls in between those spectrums, venturing into pop, jazz, soul, New Age, funk and rock to reach their means. Even at its ambitious two-LP-plus-EP length, the album rarely lags.

Songs in the Key of Life debuted at No. 1, only the third album to do so at the time (the first two were by Elton John for the pair of records he released in 1975, Captain Fantastic and the Brown Dirt Cowboy and Rock of the Westies). It also won a Grammy for Album of the Year, repeating the honors held by Wonder's two previous LPs. And it's sold more than 5 million copies along the way.

It's no surprise Wonder couldn't keep up this streak. In a way Songs in the Key of Life was his victory lap, following the legend-making decade he had up to the point. He filled out the '70s with a quasi soundtrack, Journey Through the Secret Life of Plants, before starting the '80s with the more slimmed-down but still excellent Hotter Than July. But even with the list of Wonder's achievements – a No. 1 single when he was barely into his teenage years, timeless songs made as part of Motown's formidable '60s stable, those '70s albums – leading up to this moment, Songs in the Key of Life was a once-in-a-lifetime record.

That was pretty clear almost immediately. And it's become even more so all these years later.

Stevie Wonder Albums Ranked

Was there a better run of albums in the '70s than Stevie Wonder's string of classics?