What is travers and how does it differ from shoulder-in?

Travers — also called haunches-in — is a lateral movement in which the horse travels along the track with its haunches brought approximately thirty degrees inward while its shoulders remain on the track, creating a position that is essentially the reverse of shoulder-in in terms of which end of the horse is displaced from the track. In travers, the horse is bent in the direction of travel — moving left with its body bent left, for example — with the inside hind leg stepping under and across toward the outside front leg's track, creating genuine sideways movement from behind rather than the sideways displacement of the forehand that shoulder-in produces. The horse travels on four tracks in travers, with all four legs following separate paths: outside hind, inside hind and outside fore, inside fore — from outside to inside of the track. The key difference between travers and shoulder-in is the relationship between the bend and the direction of travel: in shoulder-in, the horse is bent away from the direction of travel and the forehand is off the track; in travers, the horse is bent into the direction of travel and the haunches are off the track. Travers makes greater demands on the horse's collection than shoulder-in because the inside hind leg must carry more weight as it steps across toward the outside front leg's track — a carrying demand that is greater than the crossing without weight-shifting that shoulder-in primarily produces. Travers is typically introduced after shoulder-in is confirmed because it requires the balance and collection preparation that shoulder-in has developed, and because the horse must have sufficient responsiveness to the inside leg and outside rein from shoulder-in work before travers can be correctly executed.