Four-part a cappella can be a delight to perform, especially in the very small choir. On the other hand, it is painful to a director to conduct a train wreck. This one happened when a strong-voiced, weak-reading soprano made one of her intermittent appearances with the choir. The choir had started learning this piece of music without her.
Very often the two upper voices move in duet-at-thirds. (We’ll say “soprano” and “alto” here; the men’s choir uses different terms.) Much less often, the soprano and tenor parts duet similarly, but with a subtle twist. The duetting twist is easy to understand and be forewarned of.