text: Chicago was released in January 1970 on Columbia Records and was an instant hit, reaching number 4 on the Billboard 200 in the United States and number 6 in the UK. Columbia Records was very active in promoting its quadraphonic four-channel surround-sound format in the mid-1970s, and nine of Chicago's first ten albums were made available in quad. The quad mix features elements not heard in the standard stereo mix, including additional guitar work from virtuoso Terry Kath in "25 Or 6 To 4" and a different vocal take from Lamm in "Wake Up Sunshine," which reveals a different lyric in the song's last line.
  In 2002, Chicago was remastered and reissued on one CD by Rhino Records with the single versions of "Make Me Smile" and "25 or 6 to 4" as bonus tracks. Rhino released a DVD-Audio version of the album in 2003, featuring both Advanced Resolution Stereo and 5.1 surround sound mixes. In 2016, British producer and musician Steven Wilson remixed Chicago from the original multitrack tapes. This version was released on January 27, 2017, by Rhino Records. A vinyl edition of the remix cut by Kevin Gray was released on August 11, 2017. Robert Lamm stated in an interview that the album has been nominated for the Grammy Hall of Fame more than once.
  Musical style, writing, composition
  In a 2015 article, Classic Rock Review says the album saw Chicago's, "full immersion into mainstream success while still building on their fusion of rock, funk and jazz."