Thanks Leadfingers and Tattie Bogle. I never thought about different songs being in different chords and the instrument having to be adjusted accordingly. It's all a mystery to me but I don't (ahem) fret. If I may add, the concert was held in an auditorium built in 1930 and very ornately so. There has been no attempt to modernize so the seats are covered in the original mohair and the original stained glass windows and decorative plasterwork are still in place. The acoustics are great. It was the perfect setting for such a jewel as Joan Baez.