The shoulder-in is a lateral movement in dressage used to supple and balance the horse and encourage use of its hindquarters.
In the eighteenth century, the French riding master Francois Robichon de la Gueriniere adapted the movement for use on straight lines.
In the shoulder-in, the shoulder of the horse is brought to the inside, creating a 30-degree-angle with the rail, with the neck bent only the slightest amount, only softening in the jaw so that the corner of the eye is visible to the rider.
Thus, as the circle becomes the shoulder-in, the rider's shoulders are turned to the inside, while his/her hips remain "straight" on the track.
Variants of the Shoulder-In include the Shoulder-fore, where less angle is asked, and a four-track movement is created.