In July 1970, when Chicago performed "25 or 6 to 4" at Tanglewood, they were still riding high on the success of their second album, "Chicago," released earlier that year. This album, also known as "Chicago II," included several major hits, solidifying the band's place in the rock genre. The performance at Tanglewood showcased not only their musical talents but also the impressive guitar work of Terry Kath, whose solo on this track became iconic. The band's fusion of rock and jazz elements, combined with their dynamic brass section, really began to distinguish them from other bands of the era. This live recording is particularly noteworthy as it captures a pivotal moment in Chicago's early career, highlighting their energy and musicianship just as they were gaining national acclaim. This concert contributed significantly to the band's reputation and the enduring legacy of both Kath and Chicago's unique sound.