In 1975, John Sebastian, former member of the beloved 60s pop group the Lovin’ Spoonful, was asked to write and record the theme song for a brand-new ABC television show with the working title Kotter. As any songwriter would, Sebastian first tried working that title into his song, but somehow the rhymes he came up with for “Kotter”—otter, water, daughter, slaughter—didn’t really lend themselves to a show about a middle-aged schoolteacher returning to his scrappy Brooklyn neighborhood to teach remedial students at his own former high school. So Sebastian took a more thoughtful approach to the task at hand and came up with a song about finding your true calling in a life you thought you’d left behind. That song, “Welcome Back,” not only went on to become a #1 pop single on this day in 1976, but it also led the show’s producers to change its title to Welcome Back, Kotter.

What Sebastian’s sweet, wistful and playfully nostalgic tune did not do, however, was influence the tone and content of the show. To listen to “Welcome Back,” you’d think that Welcome Back, Kotter was a seriocomic slice-of-life program in the mold of, say, The Courtship of Eddie’s Father—another 70s TV show with a theme song by a great 60s songwriter (Harry Nilsson). Instead, Welcome Back, Kotter was little more than a flimsy platform for catchphrase-spouting caricatures, albeit an insanely successful one. Arnold Horshack’s “Oooh, oooh, oooh,” Freddie “Boom Boom” Washington’s “Hi therrre,” Vinnie Barbarino’s “What? What?” and Gabe Kotter’s “Up your nose with a rubber hose” were the pop-cultural coin-of-the-realm in 1975-76, and though they bore little relation in tone or spirit to the song that topped the charts on this day in 1976, the disconnect did nothing to hinder the popularity of all things Kotter-related. Indeed, if you weren’t wearing an Uncle Sam or King Kong T-shirt in the summer of America’s bicentennial year, you were probably wearing one with a picture of “the Sweathogs” and a colorful phrase like “Off my case, toilet face” on it.

“Welcome Back” was the first and only television theme song that John Sebastian ever wrote, but it was far from the only television theme song of the mid-1970s to become a legitimate pop hit. Only weeks earlier in 1976, the instrumental “Theme From S.W.A.T.” had topped the Billboard Hot 100, and the excellent Mike Post-written theme The Rockford Files had made the top 10 the previous summer.