25 or 6 to 4

Robert Lamm wrote 25 or 6 to 4, recorded in July 1969 for the second album with Peter Cetera singing lead. Lamm said in 2013 that he composed it on a 12-string guitar with only ten strings and wrote the lyrics in a day. The title is the time, 25 or 26 minutes before 4 a.m., and the song is about trying to write a song in the middle of the night. The band first rehearsed it at the Whisky a Go Go. Terry Kath plays a wah-wah guitar solo on the original.

Columbia edited it for single release in June 1970, omitting the second verse and most of the solo. It reached number 4 on the Hot 100 and number 7 in the UK, the band's first top-five hit in the United States. Singapore banned it in 1970 over alleged drug allusions and lifted the ban in 1993. A 1986 remake with Jason Scheff on lead was made for Chicago 18, and its video won an American Video Award for cinematography. Billboard writers ranked the original first on a 2019 list of the band's 50 best songs, and former Metallica bassist Jason Newsted said it was the first rock or metal riff he learned.