[edjw]

There aren't any accompanying instruments but singers do play other instruments in the rest of their life.

In Sacred Harp, before a song is sung, somebody sings out the notes of the opening chord of the song using a combination of ear/feel/memory/practice/experience. The same song can be keyed a bit higher or lower depending on things like: energy of the singers, time of day, whether the last song was quite high or low, how easy the highest or lowest written notes are for the singers in the room. It is a skill! But most keyers will usually pitch songs written in F major at roughly the same pitch and will pitch songs written in D major differently to F major.

You can hear it on any recording but around 0:15 here is an example: https://www.youtube.com/watch?v=y8bKu52cv6o