A horse should stand square with its cannon bone hanging straight down. If this is not happening as shown in the picture on the right, the horse's natural stay apparatus cannot function and the horse will require muscle energy to stand. As a result, problems with the horse's body can then occur.
What is a stay apparatus?
The stay apparatus is a system of bones, tendons, ligaments, fascia and muscles that work in balance with one another to allow the horse to stand with no muscle action. This is why the horse can stay standing up while it is asleep.
The Front Hoof
The toe angle of the front hooves should be about 45 to 50°. The coronet angle should be 30° to the ground and the toe/coronet angle should be about 105°. The hairline seen from the front should be parallel to the ground. The heel height should be about 3 to 4cm from the hairline of the bulbs. The front hooves hold about 60 to 80% of the horse's weight, and the hoof is rounder and flatter in concavity than the hind hoof. Concavity should be about 1cm deep at the apex of the frog.
The Hind Hoof
The toe angle should be about 50 to 55°. Coronet 30° and the coronet angle about 95°. The hairline in the front should be straight. The hind hoof is for impulsion therefore, the hoof is a bit more triangular and has more concavity than the front hoof. Concavity should be about 2 to 2.5cm deep at the apex of the frog. The hind hoof is not entirely symmetrical, the lateral (outside) quarter is slightly wider and shallower and the medial (inside) quarter is slightly straighter. This should not be extreme.
The Coffin Bone
The coffin bone is the bone that sits inside the hoof. The placement of the coffin bone inside the hoof is vital, as this affects the whole limb. The angle of the coffin bone should be between 0 to 5 degrees from the ground plane. The higher the coffin bone is raised off the ground parallel, the higher the effects on the distal (front) edge of the coffin bone which are detrimental to the laminar connection between the coffin bone and the hoof capsule. The coffin bone of any healthy front hoof has a toe angle of 45°, and the hind coffin bone angle should be 55°.
The Bottom of the Hoof
The bulbs and the last third of the frog should be the first part of the hoof that has contact with the ground. Heels should be broad and smooth with the frog as wide as possible. Bars should end at the midpoint of the frog. The sole should be smooth and vaulted. The "moon sickle" is the only part of the sole that should be bearing any weight.
Depending on the breed, the lateral walls can diverge a lot. For example, in warmbloods, Friesians and draft horses they diverge a lot whereas in hard hoofed horses such as quarter horses, Andalusians and Arabians they diverge less.