Lots of people are confused by the difference between the dressage movements shoulder-in, shoulder-fore, and first position. They all have a place as far as straightening your dressage horse. That’s because we always straighten the horse by bringing the forehand in front of the hindquarters. Never straighten the horse by pushing his hindquarters over behind his shoulders.

Let’s talk about first position first. To make your dressage horse straight, always ride him in first position.
•    First position teaches him “baby engagement” (bending of the joints) of his inside hind leg.
•    If you don’t ride your dressage horse in first position (especially when his hollow side is on the inside), he’ll open the joints of his inside hind leg and place his hindquarters to the inside. As a result, he won’t carry as much weight on his inside hind leg, and his hind legs won’t become equally strong.
•    If you don’t make it a habit to ride in first position at the walk and trot, the first time you’ll really see that your horse is crooked is when you canter. He’ll put his hindquarters to the inside because you haven’t taught him the mechanics of bending the joints of his hind legs.

Here’s one way to think of first position:
•    Shoulder-in is a straightening exercise. Bring your horse’s shoulders to the inside at a 30-degree angle to the track. At 30 degrees, his outside foreleg is in the same track as his inside hind leg.
•    Shoulder-fore is also a straightening exercise. Displace your horse’s shoulders to the inside at a 15-degree angle to the track so that each leg is traveling on it’s own track.
•    Think of first position as “1/100” of a shoulder-in. So, use a very subtle version of shoulder-in aids to get first position. First position is not an exercise like shoulder-in and shoulder-fore. It’s simply the correct position of a straight horse.

Here’s another way to think of first position:
If you’re riding in first position toward someone who is standing at the end of the long side, she can’t see the outside hind foot, because it’s hidden behind the outside front leg. But she can see half of a hoof’s width of the inside hind leg stepping to the inside of the inside foreleg.

When you’re first learning to ride your dressage horse in first position, you’ll probably override the angle and do shoulder-in or shoulder-fore. So it’s helpful to have a ground person or mirror so you can develop a feel for the subtlety of this position.

Click here for more info on shoulder-fore shoulder-in.