D.C. Code § 49-307: Filling vacancies above the grade of 2nd lieutenant.

Vacancies occurring in the cavalry, coast artillery corps, field artillery, and infantry above the grade of 2nd lieutenant shall, subject to the examination required by law, be filled by promotion according to seniority from the next lower grade in the troop, the separate company, the field battery, the separate battalion, and the regiment in which the vacancy occurs.