How do you teach a horse to stop straight?

A horse learns to stop straight by first learning to travel straight, because the stop is simply the end point of the rundown, and a horse that drifts left or right during the approach will arrive at the stop in a crooked position that it cannot correct at the moment the stop is asked. Straightness must be built progressively from the slowest gaits before it is expected at speed. At the walk, the horse should travel a straight line down the pen with equal contact on both reins and equal engagement from both legs, without drifting toward the rail, the gate, or any preferred direction. The rider must be able to control the shoulders and hips independently — moving the shoulder right to correct a left drift, moving the hip left to correct a right drift — so that straightness is actively maintained rather than hoped for. At the trot and then the lope, confirm that the horse travels straight on a loose rein before adding speed. Horses that drift consistently to one side at the lope will drift the same direction at a gallop, and the correction must happen at the slower gait where it can be applied precisely before it is expected at speed. In the rundown specifically, ride toward a fixed point on the fence rather than looking at the ground, and use both legs equally to keep the horse in a lane rather than allowing it to wander. When straightness through the approach is confirmed, the stop itself should be straight because the horse's body was aligned correctly before the stop was asked. Straightness in the stop that is corrected at the moment of the stop itself is always less reliable than straightness built into the approach.