I was riding in a Bill McMullin clinic and had my mind blown, and I wonder why nobody ever told me this before.
The shoulder-fore is not just a more shallow shoulder-in. It’s actually designed to get the horse straight on the rail. Here is how Bill explained it.
The horse is narrower in the front than they are in the back, so if you keep their shoulders right on the rail they will always look like they are doing a slight haunches-in because their butts are much wider than their chests. So, to have the inside front foot actually lined up with the inside hind foot you have to have the horse _very slightly_ angled into the ring, but _without any of the bend you would have in an actual shoulder-in_.
Thus ends dressage neepery of the day. And here is an image of a horse doing the shoulder-in.
