A crucial prerequisite to introducing extended canter on the diagonal is the flying change itself. Itís not a question of whether the horse can do a flying change, but whether the horse can do a good flying change. The canter must remain straight and through, not just before the change, but during the flying change and afterward as well. The other important prerequisite is the horseís ability to go from extended canter to collected canter without losing the connection. Problems almost always arise with the extended canter diagonal as a result of the horse becoming crooked, above the bit, or losing throughness, usually on the outside rein.Schooling the extended canter on the diagonal is more about schooling the separate parts than over-practising the exact sequence as it appears in tests. If you can go from extension to collection and ride good flying changes separately, then putting them together should not be a problem.Schooling the PiecesIf I am thinking about introducing extended canter diagonals with a horse, I am assuming the flying changes on their own are already well-confirmed. I then focus mainly on the transition from extended to collected canter. I start out on the track, riding extended canter to collected canter on the long side. As the horse collects, I should be able to maintain a shoulder-fore position without any bend. My own aids must remain supporting the lead through the collection; my inside leg and inside hip are forward, and my outside hip and leg remain back. The image I like to use is to think of two railroad tracks that remain parallel. I remain straight in my position (as opposed to twisting in my position) and my inside leg and hip stay forward as I ask the horse to collect.Once I am satisfied with the quality of the canter in the transition to collection on the track, I move the exercise to the inside track, then the quarter line, and finally the centre line. I first ride down centre line in collected canter and if I have a mirror, I want to see that the horseís head is straight and centred. If the horse is perfectly straight, I should not be able to see the hind legs in the mirror as I canter toward it. I then ride down centre line in extended canter, with a transition to collected canter as I near the end of the centre line. Mirrors are very useful for this exercise, because they give me a mental image that I can then take to the diagonal. When I progress to riding extended canter on the diagonal, I can picture what that correct, straight canter looked like on centre line, and think of the diagonal as a centre line.Avoiding AnticipationIf I am riding a grand prix horse, I would do the flying change at the end of the diagonal relatively often when I school extended canter on the diagonal. With a horse that is just learning this sequence, however, I would rarely ride the flying change at the end of the diagonal. Schooling the diagonal is about achieving the extension to collection while maintaining straightness. If I ride the diagonal and feel that the horse has remained connected and straight, I will ride a flying change at C, or on the next long side. I will try the flying change at the end of the diagonal only after I have confirmed that the horse is not anticipating it.The extended canter diagonal is like any other exercise; I break it down into its individual parts and make sure they are all really good before I put them together. Schooling the line from the tests is rarely needed, except as an occasional test to make sure you can string all the parts together.