How to achieve Straightness

Straightness - the fifth point on the scale of training dressage.

Straightness, such as rhythm, suppleness, contact and impulsion, is a requirement for every following dressage work. The horse should walk, trot and canter in a straight way. 

Shoulder-in and other lateral movements can help the horse to develop straightness.

Another option is the change from straighten the horse to bending. While riding in the arena, there a re four corners → ride them with bending. The long and short sides are good to straighten the horse. It can also be easier to ride a circle (20x20 or even smaller) and to ride straight and sometimes bend the horse.