An early lesson for me. So far back that I can't remember where or when, I learned that one should not allow a horse to lean. At the time I did not appreciate why.

I only knew that a good dressage horse was supposed to stay upright underneath the rider. For all I knew it was because they didn't want the knights to fall off with their five hundred pounds of armor.<lol>

Going on to teaching therapeutic riding again I did not question the need to train the horse to stay upright. Our riders had enough difficulties staying on, without the horse's leaning. Every circle, every corner, every gait the horse had to stay in an upright posture and bend, never lean.