Again, I think you (one -- I do this) is tempering the scale as one goes to make it sound right, and often that it just to make it right for the next few notes. I would try it with a piano or a patient friend, and I think you would be suprised how far off you can go. I did it recently and was amazed, though I was able to get back to the tonic (that seems to hang on while the rest goes off into 4 flats and has to be winched back in again). I also shift octaves when things get tough. One thing I have learned is that the flashy parts stick in the head and stay in tune with each other, but it is getting the ordinary stuff to stay in tune a cappella that is so hard. That may be just me.

I know that singing harmony the other person has a different version of how the key holds together -- they need to be able to anticipate a fraction, and they will temper to you for awhile, and then lose their temper. I have a patient teacher, but I know that he holds back his criticism. I remember someone being interviewed about Emmylou Harris who does a lot of duets with all kinds of singers, and they were wildly impressed by her patience and ability to quickly adjust to the wanderings of the singers she was with. I imagine that is why brothers and sisters sing well together -- some subsconscious synching going on.

yours, Peter T.