@Jackaroe said:

I had enough of a basic music introduction as a kid to get the basics of this article, but am by no means a musician. Question for those that are: does Phish plan these key changes before they take the stage? I know that some are standard per the song, but it is clear that they will step outside of typical progressions as well. Just curious as to how four musicians co-ordinate that in real time.

I assume that they do not plan these key changes at all (with exceptions, such as the 8/4/15 Mike's Song where Trey likely told the band that they would do a second jam in F major after the typical E minor jam with the F# minor finish). What they do is explore tonal space, and as @MikeHamad mentioned, if someone offers an idea all the rest of the band has to do is take them up on said idea. But Gordeaux does have to decide to "go there" for it to really be a modulation. This is evidenced by the fact that the same song will behave differently on any given night. Or, consider the 11/22/97 Halley's, where Trey can famously be heard on the SBD yell "Mike! Stay on F!"

I bet that maybe, in 1992 or 93 when they were still learning how to group improvise, they did plan out these transitions. But back then it was likely in the service of moving from one song to the next, because setlists were written out and the whole show was a very calculated affair.