Delaware is the only state with this
Answer: Rounded Border
Conde Nast - When the Duke of York originally granted Delaware to William Penn, he defined its top bit as “all that the Towne of Newcastle otherwise called Delaware and All that Tract of Land lying within the Compass or Circle of 12 Miles about the same scituate (sic).” In other words, Delaware was defined as a twelve-mile radius around New Castle. And it’s not just on the northern border, either. It might be too small to see in a road atlas, but there’s a small stretch of the Delaware-Maryland border, less than a mile long, that bulges west slightly due to the twelve-mile arc intersecting the boundary line there.