 The county was transferred to Massachusetts on October 7, 1691, and at the same time Nantucket Island was split into the separate Nantucket County, Massachusetts.The 1695 incorporation statute created a county "by the name of Dukes County," as opposed to the standard form "the county of Dukes" which is the reason for the redundancy in the formal name, "County of Dukes County".According to the U.S. Census Bureau, the county has a total area of , of which is land and (79%) is water.