"I have adopted the letters D & S to designate the broader categories of the shape of the horses back. This originates from Xenophon, a famous Greek horseman who was considering this problem in approximately 360 BC. Xenophon described horse’s backs in two ways, double back (D) or single back (S). With the double back, the flesh rises on either side of the spine, so that the spine lies in a slight depression. Single backs, have the spine rising above the flesh forming a ridge."