Chicago had never had a No. 1 single before Oct. 23, 1976. That week, “If You Leave Me Now” replaced Rick Dees’ “Disco Duck” at the top of the Billboard Hot 100. It stayed there for two weeks before the Steve Miller Band’s “Rock’n Me” took over, according to Billboard’s 1976 chart history .

Finished to Fill Out the Album

Peter Cetera, the band’s bassist, wrote and sang “If You Leave Me Now” for “Chicago X.” Columbia released it as a single on July 30, 1976, per the song’s history .

It almost didn’t make the record. Band members said later that the song was finished at the last minute so the album would have enough tracks, Ultimate Classic Rock reported . Producer and manager James William Guercio played bass and acoustic guitar on the early version as placeholders, and his parts stayed on the final take.

Saxophonist Walter Parazaider barely knew the finished song. The first time he heard it on the radio, he thought it was a new Paul McCartney single.…