The vast majority of physicians trained in the United States have a Doctor of Medicine degree, and use the initials M.D. A smaller number attend osteopathic medical schools and have a Doctor of Osteopathic Medicine degree and use the initials D.O.[10] The World Directory of Medical Schools lists both MD and DO granting schools as medical schools located in the United States. After completion of medical school, physicians complete a residency in the specialty in which they will practice. Subspecialties require the completion of a fellowship after residency.