Residency training programs are either four or six years in duration. In the United States, four-year residency programs grant a certificate of specialty training in oral and maxillofacial surgery. Six year programs granting an optional MD degree emerged in the early 1990s in the United States. Typically, Six-year residency programs grant the specialty certificate and an additional degree such as a medical degree (e.g., MD, MBBS, MBChB) or research degree (e.g., MS, MSc, MPhil, MDS, MSD, MDSc, DClinDent, DSc, DMSc, PhD). Both four– and six–year graduates are designated US "Board Eligible" and those who earn "Board Certification" are Diplomats. Approximately 50% of the training programs in the US and 66% of Canadian training programs are "dual-degree." The typical total length of education and training, post-secondary school is 12 to 14 years. Beyond these years, some sub-specialize, adding an additional 1-2 year fellowship.