Look at a map of the United States and find the seam where Delaware meets Pennsylvania.
Every other state boundary in that part of the country is doing one of two sensible things: following a river, or running straight. Delaware's northern border does neither. It curves — a smooth, deliberate arc of a circle. The National Park Service, which now interprets part of the boundary through First State National Historical Park, calls it simply "unique among U.S. state boundaries," and no geographer has ever pointed to a second example. As far as this publication can determine, it is the only state boundary in the country shaped that way.
It is not a mistake, exactly. It is the physical residue of an argument.
Twelve miles from New Castle
The argument began with the standard problem of English colonial America: the crown kept granting the same land to different people.
In 1682, the Duke of York — who had controlled the lower Delaware since seizing it from the Dutch in 1664 — transferred that territory to William Penn, who had received the charter for Pennsylvania from Charles II the year before. Concerned about protecting what he still held from Penn's new colony, the Duke of York had already directed that a boundary be drawn as a circle with a twelve-mile radius, centered on the town of New Castle.
A circle. Not a line twelve miles north of some point — an arc twelve miles out, in every direction.
On paper it was a clear instruction. In the field, in 1681, it was something close to absurd. Surveying a straight line through forest and swamp with chains and compasses was hard enough; closing a true twelve-mile-radius arc correctly was harder still. Complicating everything further, the Calvert family of Maryland held an earlier, overlapping grant, and the resulting dispute between the Penns and the Calverts ran for generations through English courts.
Drawing a circle in the woods
The first serious survey attempt came in 1701, when Isaac Taylor of Chester County, Pennsylvania, and Thomas Pierson of New Castle County were appointed to run the boundary. What they produced was an arc running roughly 120 degrees westward from the Delaware River, terminating due west of New Castle — an impressive piece of work for the instruments available.
It was not immediately accepted by either side. Pennsylvania did not approve the survey until 1715; Delaware did not approve it until 1719. By the mid-eighteenth century, the circle's formal center point had been fixed at the cupola of the New Castle Court House — a specific architectural feature of a specific building, functioning as the geometric origin of a state. The building still stands, and the cupola is still there.
Mason and Dixon
In 1763, the Penn and Calvert families, exhausted by litigation, hired two Englishmen to settle the matter with better instruments than anyone had previously brought to it. Charles Mason was an astronomer; Jeremiah Dixon was a surveyor. Together they produced the most precise colonial-era survey in North America, and their names later attached themselves to a line whose political meaning they never intended.
Their Delaware work is the least famous and most technically interesting part of the job. Using the New Castle Court House as the center point, they had to reconcile three separate lines: the Tangent Line, running from a point on the Transpeninsular Line to a point exactly tangent to the Twelve-Mile Circle; the North Line, running north from that tangent point; and the Arc Line — where the trouble was.
When Mason and Dixon completed the Tangent Line and ran north, they found that the tangent point sat somewhere other than earlier surveys had placed it, and the North Line consequently cut through the Twelve-Mile Circle rather than running clear of it. Their fix was to let the boundary follow the Circle itself for about a mile and a half from the tangent point, until the arc met the North Line. That short segment is the Arc Line, and historians commonly put the sliver of extra territory it added to Delaware at around thirteen acres — two of the finest surveyors of the eighteenth century, several years of work, and an outcome that hinged on a parcel roughly the size of a large farm field.
The Wedge
The North Line's misalignment created a second, longer-running problem. Because the Twelve-Mile Circle did not meet the Maryland–Pennsylvania line the way the original grants assumed it would, a small triangular area was left over at the point where the three jurisdictions converge — claimed by Pennsylvania on one reading, by Delaware on another. It became known as the Wedge: reported variously at a bit over one square mile to around 1.25 square miles, depending on the source, which itself is a small demonstration of how imprecise this entire boundary business was from the start.
For most of the nineteenth century the Wedge sat in genuine jurisdictional limbo. The people who actually lived there resolved the practical question the way people usually do: they voted in Delaware and paid their taxes to Dover. The formal resolution, once it finally moved, followed a documented sequence: a joint boundary commission awarded the Wedge to Delaware in 1889; Pennsylvania ratified that decision in 1897; Delaware ratified in 1921; and Congress gave its approval the same year, by joint resolution that June. Two hundred and forty years after the original 1681 grant, the top of the Delaware map was finally settled.
What "the circle" actually means
A few things get garbled in the popular retelling. It is not a full circle — it is an arc, a portion of a circle, interrupted on the east by the Delaware River. The center point is not the town of New Castle in general; by the eighteenth-century settlement, it is a specific building's cupola. And the boundary as it exists on the ground today is not a mathematically perfect twelve-mile-radius circle; it is the product of eighteenth-century instruments, later adjustments, and the Arc Line correction, smoothed into the line shown on modern maps, as the National Park Service's own account puts it.
One more oddity follows directly from the same seventeenth-century grants: within the Twelve-Mile Circle, Delaware's jurisdiction extends across the Delaware River to the low-water mark on the New Jersey shore, rather than to the river's midline as most state boundaries on rivers do. That has been the subject of real litigation over the years and is a story of its own.
The stones are still there
Mason and Dixon set physical markers, and many of them are still standing today, including crownstones — more elaborate markers placed at intervals, carrying the coats of arms of the Penn and Calvert families on opposite faces. They sit in fields, at roadsides, behind fences, in the ordinary landscape of the Delmarva Peninsula, doing the job they were cut for in the 1760s. The Wedge's tripoint is marked. The New Castle Court House still stands in the middle of New Castle, with the arc still radiating, on paper, from its cupola.
None of it is behind glass. It is a set of ordinary-looking stones in ordinary-looking places, holding a state's shape together, because in 1681 a duke made an arrangement nobody could easily survey, and it took until 1921 to finish arguing about it.
Sources: U.S. National Park Service, "Delaware's 12-Mile Arc" (nps.gov); Delaware Division of Historical and Cultural Affairs, "Surveying the Landscape of the 'Delaware Wedge'" (history.delaware.gov, 2023); secondary historical accounts of the Mason–Dixon survey and the Wedge boundary commission. The Wedge's exact acreage is reported inconsistently across sources and is given here as a range rather than a single figure.