What do the letters after a doctor's name stand for?

The letters after a doctor's name indicate the advanced degree program the doctor completed, according to About.com. The most common letters are M.D. for medical doctor and Ph.D. for doctor of philosophy.

Other post-nominal letters for doctors, according to About.com and Wikipedia, include D.D.S. for doctor of dental surgery, D.M.D. for doctor of dental medicine, O.D. for doctor of optometry, Au.D. for doctor of audiology, D.P.M. for doctor of podiatric medicine, D.P.T. for doctor of physical therapy, D.N.P. for doctor of nursing practice, D.C. for doctor of chiropractic and PharmD for doctor of pharmacy. A doctor of veterinary medicine uses the initials D.V.M. after her name.