Bone. As in the refers to a horses leg
This is the measurement of the circumference around the horses cannon bone about halfway between the knee and the fetlock joints. Eight inches of bone is the average for a Thoroughbred. “flat bone” indicates that the cannon and the back tendon are parallel, with the tendon clean-cut and standing well away from the cannon bone. The word “flat” refers to the appearance of the cannon, which is wide and flat when viewed from the side although narrow from the front, and does not mean that the bone itself is flat.