What is the difference between bend and flexion in dressage?

Bend and flexion are related but distinct concepts in dressage that describe different dimensions of the horse's lateral alignment, and confusing them produces incorrect training that develops one without the other or conflates the two in ways that create evasions. Flexion specifically refers to the lateral softening of the horse's jaw and poll — the yielding of the horse's jaw to the inside rein and the slight turning of the horse's head so that the rider can just see the horse's inside eye and nostril when looking forward between the horse's ears. This flexion at the poll and jaw is a prerequisite for correct bend but is not the same as bend: a horse can flex at the poll without bending through its body, which is the common error of neck bend without body bend, and a horse can have correct bend through its body without excessive flexion at the poll if the bend is being maintained primarily through leg and body rather than rein aids. Bend describes the uniform curve of the horse's spine through its entire length from poll to tail, following the arc of the circle or curved line with the inside hind leg stepping under the body toward the outside front leg's track. Correct bend requires both the flexion at the poll that allows the horse's head to be positioned correctly within the bend and the lateral flexibility of the spine and ribcage that allows the horse's body to follow the curved path. The practical significance of the distinction is that developing genuine bend requires primarily leg and body aids — the inside leg creating the bend, the outside aids containing it — while flexion is developed more directly through rein contact. Asking for flexion without bend produces a horse whose neck curves but whose body remains straight; developing bend through leg and body without adequate attention to flexion produces a horse that is stiff through the poll and jaw despite moving on a curved line.