By Archtop (Special thanks to member Paperback Writer at the Steve Hoffman forums).
The jam commonly referred to as the Tighten Up jam is so referred to based on the chords to (and its overall resemblance to) the 1968 track of the same name from Archie Bell & the Drells.
The problem herein lies in the chord structure of the jam. The original composition of Tighten Up is based on a simple chord change of F#Maj7 – BMaj7 (or, if you prefer to make your life more difficult, GbMaj7 – CbMaj7). In any case, these two sets of chord changes represent I IV in a Major key. But the GoGD jam in question is not I IV in a Major key; rather, it is I ii in a Major key. See 5/2/70, DP 8, Dancin’ in the Street from 6:52-10:15 (all times taken from the official releases noted herein)
and 10/31/71, DP 2, Dark Star from 13:29-17:52;
for officially-released and well-known versions. But often, things are not so cut and dried with the GoGD. The 1/2/70, DaP 30, Dark Star complicates things by including an unnamed jam starting at 19:41–> Feelin’ Groovy (starting at 21:42 and then meandering away)–> Soulful Strut (transposed; starting at 23:40).
Keep in mind that any transposition our boys might have done is irrelevant here. Transcribing a set of chord changes to another key does not affect their harmonic function.
However, the Grateful Dead play the Soulful Strut jam in A Major, using A and Bmin as the two chords. This represents both a transposition from F#/Gb Major and a reharmonization of I IV in a Major key to I ii in a Major key. But if we look at Soulful Strut, a 1968 composition by Young-Holt Unlimited, the main set of changes is BbMa7 – Cmin7: I ii in the Major key of Bb Major.
Don’t get hung up on the key/tonal center; focus on the functionality of the chords. If we transpose the two chords from Soulful Strut down a half step, we get A Bmin. Thus, what has long been known as the Tighten Up jam is, by its very harmonic construction, much more close to Soulful Strut than it is to Tighten Up.