BILL— In dressage seat equitation there is mention of the posting diagonal, nowhere else in our discipline. Consequently there is no such thing as an “incorrect diagonal.” In several books of eastern European origin a theory is even espouse that riders should rise on the inside diagonal.
If, however, you only post on one diagonal throughout your entire test, a judge may be suspicious that you are trying to hide a lameness and will dock your Rider score in the Collectives.
Where you switch your posting diagonal is up to you, the most common place being where the horse’s bend is changed. If in Training Level you change at X you will not be scolded. In First Level when making a lengthening it is better to do it at the corner you’re arriving at so as not to disrupt the rhythm In the middle of the arena.