How do I learn diagonals?

Learning diagonals is one of the first genuinely technical riding skills that new trot riders encounter, and it is one of those skills that feels impossibly abstract in the first few lessons and then suddenly clicks into a pattern that becomes automatic within a few weeks of consistent practice. Getting there efficiently requires both understanding what diagonals actually are and why they matter, and developing the specific feel and visual check habits that allow the correct diagonal to be identified and corrected reliably before the automatic feel develops. The diagonal in posting trot refers to which pair of the horse's diagonal legs the rider is rising with during the posting motion. In a trot the horse moves his legs in diagonal pairs — the left front and right hind move together as one diagonal pair, and the right front and left hind move together as the other. The correct diagonal for riding on a circle or on a track is to rise as the outside front leg moves forward — which means you are sitting as the outside hind leg is on the ground and pushing, which is the hind leg that needs to carry the most weight and work the hardest on a circle. The visual method of checking the diagonal — looking down at the horse's outside shoulder and rising as it moves forward — is the standard teaching tool and the most reliable method for riders who have not yet developed the feel for the diagonal from the seat. The outside shoulder is the one on the side of the arena fence when riding on a track, or the shoulder on the outside of any circle being ridden. Watch that shoulder as you post and confirm that you are rising when that shoulder moves forward and sitting when it moves back. If you find that you are on the wrong diagonal, the correction is a single extra sit — sitting for two beats in a row rather than one — which shifts you from one diagonal to the other. The feel method of identifying the diagonal is what replaces the visual check as riding skill develops. The feel of the correct diagonal is a sensation of being carried forward by the horse's outside shoulder and outside hind leg — a pushing forward sensation through the correct diagonal that is slightly different from the feel of posting on the wrong diagonal. Developing this feel requires deliberate practice — look at the diagonal to confirm the correct one, then close the eyes briefly and try to feel the specific quality of the movement beneath you, then open the eyes to check. Repeating this look-feel-check sequence over many rides progressively develops the association between the visual confirmation and the physical feel until the feel becomes reliable enough to identify the diagonal without looking. For training purposes the general recommendation is to alternate diagonals on straight lines — posting on the left diagonal for one long side and then changing to the right diagonal for the next — to develop both diagonals equally in both horse and rider and to prevent the one-sidedness that consistent posting on a single diagonal produces over many rides.