North of the Frozen Tundra, but I can see it from my house.
No rule as such, but still don't be caught posting on the wrong one. I have changed at X and not found it upset the trot. It would probably be better to wait until the end of the diagonal. Just don't ever be caught on the wrong one.
So long as your horse is balanced, you should not be penalized for posting on the wrong diagonal.
I've never done it in a test (because, really, it's usually harder to have a horse balanced on a circle if you aren't on the correct diagonal, and I don't need to make things harder for myself! ), but I've done it in rail classes (not equitation, of course) on a horse who went smoother on one diagonal than the other - and won. While I can't speak for every judge, of course, it's not nearly the taboo people would believe - again, so long as it doesn't negatively effect the horse's performance.
There is no 'wrong' diagonal (since different countries/schools have different ideas about it), so you can post on any diagonal you want. Best idea is to change (if you want) at the end of the diagonal (imho).
My daughter has been competing in dressage for 12 years. She is always on the wrong diagonal--she just posts with the one that feels the best. She has her USDF Silver Medal. No one has ever mentioned it, and our trainer, an FEI judge, says no one ever should.
\"I refuse to engage in a battle of wits with someone who is unarmed.\"--Pogo
There is no "right" or "wrong" diagonal in dressage. If you consistently post on one diagonal without switching, the judge may comment on that because you are contributing to potential unevenness. But a judge should never say you are on the wrong diagonal.
The only thing that I've learned is that its not wrong, but it can throw your horse off balance if it is a green horse learning to balance and bend. Also on my circles I just feel weird on the wrong diagnol Not so much on the rail but when I have to bend to centerline or circle I feel like it throws myself and my horse off.
Horses aren't our whole life, but makes our life whole
if the horse is even on both hind legs and is supple he feels equally well on either diagonal on a circle, it is an exercise some trainers have one do alot to change diagonals over and over on a circle.
Coming from a hunter background, I can't stand seeing someone on the wrong diagonal. (In the US, you should be on the outside diagonal, is it different in other countries?). I also spent the first half of my riding career being drilled to ALWAYS be on the correct diagonal. We would have lessons where we were not allowed to look down and had to ask for the trot and pick up the correct diagonal and know which diagonal we were on.
I have a problem with one of the first level tests where you are asked to extend the trot across the diagonal, specifically using a rising (posting) trot....I ALWAYS want to switch at X even though we are only posting in a straight line across the diagonal. It takes a lot of work for me not to and I don't because I figure it might mess up her extension and in that instance look sloppy.
As far as being penalized, some people are sticklers for details more than others. Being on the correct diagonal might be one of their peeves. If they are not officially allowed to penalize for it, they may end up taking 2 points off instead of one for a movement that is not as crisp AND the diagonal is wrong.