My periodontist says that he is "board certified." What does that mean?

All periodontists must complete an additional two to three years of specialized training in periodontics following dental school. However, some periodontists opt to take the board-certification examination, which is offered by the American Board of Periodontology once per year. Board certification in periodontology denotes someone who has made significant achievements beyond the mandatory educational requirements of the specialty, including demonstrating a comprehensive mastery of all phases of periodontal disease and treatment and in the placement of dental implants. Recertification is required every six years.