What are four-tempi, three-tempi, two-tempi, and one-tempi changes in dressage?

Tempi changes describe the pattern of flying changes executed at regular intervals across the diagonal or long side of the arena, with the number describing how many canter strides occur between each change. Four-tempi changes require a flying change every four strides — the horse canters four strides, executes a change, canters four more strides, changes again — creating a pattern across the diagonal that typically produces four to five changes depending on the arena size and the horse's stride length. Three-tempi changes require a change every three strides, producing a more frequent pattern and demanding greater balance and straightness because the horse has less time between changes to reestablish its rhythm. Two-tempi changes require a change every two strides — one stride on each lead before changing — creating a rhythmic sequence that appears to the observer as nearly continuous changes and demands high collection and significant responsiveness to the aids. One-tempi changes, the most demanding of all, require a flying change on every single stride — the horse changes lead with every canter stride, producing the spectacular zigzagging movement of alternating leads that is one of the most visually impressive movements in Grand Prix dressage and one of the most athletically demanding. In Grand Prix tests, riders are required to perform fifteen consecutive one-tempi changes, which demands that each individual change be completely clean and that the horse maintain perfect rhythm, straightness, and balance through all fifteen changes without any deviation. The one-tempi changes require maximum collection because each stride must be short and uphill enough to allow the suspension phase in which the change occurs to be accessible at every stride — a flat, running canter cannot produce the suspension required for reliable one-tempi changes regardless of how much the horse has been drilled on the pattern.