The Twelve-Mile Circle is a arc that makes up the majority of the north-south boundry between the states of Pennsylvania and Delaware in the United States.

It is a circle with a twelve-mile radius, with the center of the circle in the center of the town of New Castle, Delaware. In 1750, the center of the circle was fixed at the coupola of the courthouse in New Castle. The twelve-mile circle continues into the Delaware River. It is the only territorial boundary in the United States that is a true arc.

Its existence dates from a deed to William Penn from the Duke of York on August 24, 1682, which granted Penn "all that the Towne of Newcastle otherwise called Delaware and All that Tract of Land lying within the Compass or Circle of Twelve Miles about the same scituate lying and being upon the River Delaware in America And all Islands in the same River Delaware and the said River and Soyle thereof lying North of the Southermost part of the said Circle of Twelve Miles about the said Towne."

The fact that the circle extends into the Delaware River makes for a fairly unique territorial possesion. Most territorial boundaries that follow watercourses split the water course bwteen the two territories by one of two methods, either by the mipoint of the watercourse (the Grotian Method, after Hugo Grotius) or, more often, midpoint of the the main flow channel, or Thalweg. However, due to the text of the deed, within the Twelve-Mile Circle, all the Delaware River to the low-tide mark on the east (New Jersey) side is territory of the state of Delaware.

New Jersey has often debated this claim, as the rest its terrotrial boundaries along the Delaware River are determined by the Thalweg method. The dispute has been brought to the Supreme Court of the United States on several occasions, most notably in STATE OF NEW JERSEY v. STATE OF DELAWARE, 291 US 361 (1934), and finally in STATE OF NEW JERSEY v. STATE OF DELAWARE, 295 US 694 (1935). The court's opinion for the former case contains an extensive history of the claims to this territory, and the latter contains a clause which enjoins New Jersey from ever disputing this claim again.

Related to the Twelve-Mile Circle is The Wedge.

References

• STATE OF NEW JERSEY v. STATE OF DELAWARE, 291 US 361 (1934) http://laws.findlaw.com/us/291/361.html
• STATE OF NEW JERSEY v. STATE OF DELAWARE, 295 US 694 (1935) http://laws.findlaw.com/us/295/694.html