To keep your shoulders level and your spine centered over your horse…
Think of expanding your collarbones sideways as
if the top of your shoulders were pointing East
and West on a compass. From this position, with
your rib cage back, you can elongate your neck and
spine even more into superior-torso alignment. This
well-balanced, upper-body position allows you to carry
yourself better so your horse can carry you.—Holly Mason

Holly Mason is a dressage instructor and author of the book
It’s Never Too Late and the DVD “Focus on Flexibility.”
Having studied with many modern dressage masters, she has
coalesced the biomechanical concepts of the classical principles
into teaching methods designed for contemporary riders
(dressagebydesign.com).