What is meant by diagonals in horse training and riding?

Diagonals in riding refer to the paired movement of the horse's front and hind legs at the trot — the two-beat gait where the legs move in diagonal pairs simultaneously. At the trot, the left front and right hind move together as one diagonal pair, and the right front and left hind move together as the other. Understanding diagonals — which diagonal is which, how to feel them from the saddle, and why riding on the correct diagonal matters for the horse's development — is one of the first technical skills an English rider must master and one that has meaningful training implications that go beyond simple riding convention. The two diagonals are named for the front leg whose landing the rider sits down on. Riding on the right diagonal means the rider sits when the right front and left hind land together, and rises when the left front and right hind land together. Riding on the left diagonal means the rider sits when the left front and right hind land together, and rises when the right front and left hind land together. In posting trot, identifying the diagonal requires either looking down at the horse's shoulder to see which one advances when the rider sits, or developing the tactile feel that allows the rider to identify the diagonal from the movement of the horse's back beneath them — the more refined and ultimately more useful skill. The correct diagonal for circle work is the outside diagonal — the one on which the rider sits when the outside front and inside hind land together. This convention exists for a biomechanical reason: the inside hind leg is the one that must work hardest on a circle, stepping more deeply under the body to support the bend and balance the horse through the curve. By rising when the inside hind swings forward and sitting when it lands, the rider's weight assists the inside hind's engagement rather than interfering with it. Riding on the incorrect diagonal — sitting when the outside hind lands rather than the inside — places the rider's weight on the working hind leg at exactly the moment it needs to swing freely, which reduces the horse's ability to engage and bend correctly. Changing diagonals — switching from one to the other — is accomplished by sitting for two beats instead of one, which puts the rider on the opposite diagonal. This is done every time the direction of travel changes, so that the horse is always worked equally on both diagonals. Consistently riding on the same diagonal through an entire training session creates an asymmetric loading of the horse's back and hindquarters — one hind leg works harder than the other, one side of the back is loaded more than the other — which over time contributes to the one-sidedness and uneven muscular development that trainers spend considerable time addressing. Consistent diagonal changes ensure the development work is distributed symmetrically across both sides of the horse's body. In western riding and disciplines that use sitting trot rather than posting trot, the concept of diagonals as riding technique is less explicit, but the underlying principle — that the inside hind leg should be the working leg in circle work and that both hind legs should be developed equally — is equally applicable. Western trainers who are aware of diagonal loading use exercises that balance the work between both directions and both hind legs, even if they do not talk about it in the same language as English riders. For dressage riders developing piaffe and passage, the diagonal pairs become explicitly visible and are individually managed through timing of the aids — the trainer applies or releases aids at specific moments in the diagonal sequence to influence the quality, timing, and engagement of each pair. At this level, diagonals are not just a technical riding convention but a fundamental framework for all advanced movement development.