This sextet release, an important marker in Sam Rivers’ creative evolution, has a somewhat confusing history. Recorded in early 1967, it was shelved until the mid-’70s when it came out as part of a double LP called Involution. The proper Dimensions & Extensions didn’t see the light of day until 1986, but all the same, it stands as a Rivers classic, his fourth and final for Blue Note before the switch to Impulse. Using a four-horn lineup with no piano, Rivers plays tenor and soprano saxophones and flute alongside Donald Byrd on trumpet, Julian Priester on trombone, and James Spaulding on alto sax and flute. Bassist Cecil McBee and drummer Steve Ellington find just the right ecstatic vibe in the rhythm section. (Ellington played on Rivers’ previous release, A New Conception.) Immediately on the opener, “Precis,” Rivers’ language in writing for the horns couldn’t be more striking: It’s dissonant and biting and tonally ambiguous, yet finely calibrated with a range of warm blends and colors. The rhythm is firmly rooted in ultra-modern swing, on “Precis” as well as the more Afro-Latin-tinged “Paean” (a great soprano sax feature) and the more aggressively free-form “Effusive Melange.” With “Involution” Rivers strikes a major contrast—a two-flute odyssey with Spaulding, swinging and harmonically wide open with just the rhythm section and no trumpet or trombone. He’s back on tenor, with that distinctive warm fuzz in his sound, for “Afflatus,” an inspired, out-of-tempo trio invention with just bass and drums (somewhat prescient of the sound of his later trio with Dave Holland and Barry Altschul). “Helix” ends the album with the full ensemble at a somewhat more relaxed swing tempo, again with Rivers’ attention-grabbing horn orchestration front and center. Fast-forward to 1999 and the albums Inspiration and Culmination with the Rivbea All-Star Orchestra, and one might hear a certain echo of Dimensions & Extensions in terms of the angular voicings, flexible interplay, and overall density.