Steve Provizer reviews Vic Hobson, Creating the Jazz Solo: Louis Armstrong and Barbershop Harmony, University Press of Mississippi.
Armstrong and several of the other musicians explicitly say that this was the technique that instrumental bands would also use to learn new material. The part being taken by a given instrument was dictated by the range in which it played: tuba or string bass doing the bass, the baritone part being carried by trombone and the two tenor parts by trumpet and clarinet.
[...] When I listen to early Armstrong music, or read transcriptions, I tend to filter my reactions through received wisdom about why particular notes were played. That is, I accept the primacy of the blues scale and the traditional presupposition that a “European” system of harmony was, in some way, guiding the musician through a solo. [...]
As simply as I can put it: There were techniques of what is called voice-leading that are applied in barbershop harmony. These are the rules that determine what the next note can be, bearing in mind all the other notes in the chord that have to be taken into consideration. I assume that these techniques were predicated on some combination of what sounded good in that era and what was feasible for someone to sing. In a number of cases, this meant choosing notes that would not be correct in a standard harmonic sense. Such notes might be “out of” the chord as then understood and/or create dissonances that were supposed to be avoided. “Snakes,” “swipes,” “funnel harmony” are some of the terms used to denote the moves that a voice could or would make.
Hobson does close analyses of Armstrong solos in “Copenhagen,” “Heebie Jeebies,” “Potato Head Blues” and other tunes in order to show the reader that many of the choices Armstrong makes are rooted in his considerable background in barbershop singing. He comes up with ample quotes from Armstrong to back up the idea that he was not playing the chord, per se, but how the chord sounded to him and what that sound led him to play, as filtered through his long experience with close harmony singing. I had, in fact, often wondered about certain note choices that Armstrong made in his solos and this explanation helped clarify some of these choices for me.