Strikingly, pitchers at singing conventions generally do not use any mechanical aid, such as a pitch pipe or tuning fork, to help them find the right pitch.[6] (Such aids may be more common at small local singings.)[7]
How Sacred Harp pitchers (who generally do not possess perfect pitch) achieve their ends without mechanical help is not a fully understood question. It probably helps that pitchers typically know the songs very well, and that they have the opportunity to test out how a particular key “feels” when they sing the first note aloud. Sometimes a pitcher will try one opening note, find it unsatisfactory, then execute a glissando to a neighboring pitch.
Cobb lists some ways in which pitchers make up for the lack of a pitch-giving device.[8] Some use their own voices as a kind of reference, for instance by knowing the lowest note they can comfortably sing. Others have a kind of “reference song”; a song so familiar that when they summon it to mind it is in the original key, which then can be used as a reference point.[9] For many singers, however, good pitching seems to be a purely intuitive activity, a skill they possess but cannot explain. One experienced pitcher told Cobb “it’s kind of like learning to fix an automobile–you just got to have a knack for it.”
Source: Pitching Sacred Harp music – Wikipedia
Tim B.
https://www.his.com/~sabol/SHhistory.html