Dressage: what are the rectangle letters for?

What the letters in the dressage rectangle are for A, K, V, E, S, H, C, M, R, B, P, F, D, L, X, I, G–these letters are a real nightmare for every dressagist!Those who practice this fine discipline, whether on horseback or with bindings, must confront these enigmatic symbols every day. Dressage (from French: […]