I was always told that the narrowest tree was a QH tree- a tree that is somewhat 'obsolete' so to speak, now that horses are generally wider, not the scrawny things of days gone by. Next up would be a semi-QH bar, the middle size with (approximately) the same pitch as a QH tree. The widest would be a full QH bar, with a flatter pitch than either of the other two, and wider as well. I was told this by a horsewoman with years of experience, showing up to worlds for QH. It is still subjective information though, I think, and terminology may differ even by region. Hope I helped?