My teacher sits on a folding chair at the end of the bench. This is far from ideal. For one thing, I have to skew the music very much to the right on the music rack so she can see it, which means I am always looking right when I play.
My teacher uses a studio -- one of about six -- at a large music store. The piano and setting are thus supplied by the store. It is possible she cannot control the choice of a bench.
Another problem -- when she wishes to hit a few notes to illustrate something, it is very difficult for her -- she has to stand up and reach over.
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Totally different spin on the question -- what if both the teacher AND the student are extremely obese? Say -- 300 lb each? I suppose this could happen.
Two issues --
1) Could both their fannies fit on the bench?
2) Is the bench warranted to hold 600 lbs? Perhaps if both were hitting fortissimo chords at the same time, a catastrophe could occur.
This is my wisdom on the topic . . . . .
Alzado