Because the bottom of the coffin bone can be flattened through a variety of  unnatural causes.

One should never try to pare the live sole to create concavity as there may be very little.

Understanding how to distinguish the live sole plane at the apex dirtline will help to judge

sole thickness and also concavity.

     Concavity in the sole is created by the shape of the source the sole grows from.  The bottom

of the coffin bone.

The two coffin bones below are the same height from toe to the top of what is called the

extensor process.   The coffin bone on the left is from a horse and the one on the right is

from a draft.  Understanding that the coffin bone for the draft is wider and the wings of

the coffin bone extend further back, but the height is the same as the horse helps in

understanding that the capsule is only a little taller.

The top coffin bone is from a front foot.  You can see that it is circular and that is what makes the front foot a round foot.

The bottom coffin bone is spade shaped because of the type of movement the hind hooves provide.

When looking at these two coffin bones look at the joint that should be center and you will notice that they have become

offset.  I believe that when foals are not trimmed and balanced the soft joints distort. 

The coffin bone above on the left is from a hind foot and the one on the right is a coffin bone from a front hoof.  There is only a few degrees

between the angles of  the individual coffin bones in regards to the front hoof variation and  a few degrees difference in most hind feet..

Angles of the capsule mirror the angle of the coffin bone.  When trimming the capsule for a 1/3 2/3 balance and the heel is correct height

the angle will vary about 3 to 5 degrees for either front or hinds..

Below are the bones of a new born.  You can see that the coffin bones are like arrowheads.  The bones are soft and begin to change in shape

 and density, through movement and concussion.  The foal needs to have a variety of terrain and freedom to run and pound those feet to provide

a lifetime of bone health.  Because the coffin bones are shaped as they are below the foals little hooves also reflect this shape.

Below is the coffin bone and capsule of a club foot.  The club foot  coffin bone is shaped somewhat bull nosed.  And as with the sole

being more vaulted that is because the club footed coffin bone is more vaulted.