By the end of 1970, the members of Chicago had released two double albums in as many years, and were coming off a lengthy, grueling tour. Somehow, they still managed to find time to deliver yet another two-LP set that explored fresh musical ambitions while propelling them to further commercial heights.

Chicago III arrived in stores on Jan. 11, 1971, just a little less than a year after the band's previous release, and collected more than 71 minutes of music over a sprawling four-sided effort that ran the gamut from the bluesy, funk-fueled opening track, "Sing a Mean Tune Kid," to the more experimental sounds of cuts like the instrumental passage "Free Country."

Keyboardist and vocalist Robert Lamm, who'd exerted the strongest songwriting influence on the band's first two LPs, started to yield some of that control with Chicago III. Whether it was due to exhaustion or simply a surfeit of strong material from the other songwriting members of the group, the new album's more democratic approach gave Chicago's sound an added eclectic edge while still leaving each player plenty of room to shine.

This new expansiveness was evident as soon as the needle dropped on "Sing a Mean Tune Kid," which highlighted some of the band's most soulful vocals (including some suitably nasty howling from bassist and future adult-contemporary kingpin Peter Cetera) on top of a percussive horn chart and some inspired noodling from Lamm before melting down into an agreeably cacophonous nine-minute jam, setting the tone for the ensuing hour.

Listen to Chicago's 'Lowdown'

Chicago's undimmed ambitions were also apparent after a glance at the track listing, which roughly divided Chicago III into lengthy multi-movement pieces: Lamm's "Travel Suite," inspired by the boredom, loneliness, and beauty of the road, took up Side Two, followed by guitarist and singer Terry Kath's "An Hour in the Shower" on Side Three, and concluding with "Elegy," a 15-minute-plus, ecologically minded composition contributed by trombonist James Pankow. Any album with this many items on the agenda can't help but feel a little shaggy at times, and Chicago III is, admittedly, not the tightest entry in the group's catalog. But even if it indulges in a number of musical detours, the record rarely feels indulgent — and arriving at a moment when the band members must have been teetering on the brink of exhaustion, it brims with an impressive vitality.

The band's efforts carried over to the charts, where Chicago III rose to No. 2, and on the radio, where the singles "Free" (cherry-picked from "Travel Suite") and "Lowdown" (co-written by Cetera and drummer Danny Seraphine) cracked the Top 40. They might have been in the midst of a draining stretch of recording and touring, but in terms of having room to let their musical freak flag fly while still scoring hits, they were managing a terrific balance between having their cake and eating it.

Of course, every meal has to end sometime, and after their next effort, the live release Chicago at Carnegie Hall — released later in 1971 — the group started settling down into more compact, pop-oriented material with 1972's Chicago V, further cementing their status as one of the most popular American bands of the '70s while drifting further from their experimental, classically trained roots. Decades of platinum sales still waited on the horizon, but in terms of pure ambition, Chicago III might mark the apogee of Chicago's willingness to push the boundaries of rock without regard for the patience of its audience.

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