Do you ever have the experience in rehearsal where people are singing the right note, but it's sitting just a shade too high or too low for the chord to gel? If you spend any time at all working a cappella, I bet you do (if you don't, you lead a charmed life).
I’m not talking about your regular, run of the mill tuning issues here, caused by tiredness, habit or faulty vocal production. I’m talking about a specific kind of fault where an ensemble that is basically in tune horizontally doesn’t always nail the vertical tuning.
Now, you can address this problem at an analytical level, asking people to nudge their note up or down a bit to get it into true. But this approach has drawbacks: