This plot of land, which measures just over a square mile, might be called the Pennsylvania Wedge had the boundary dispute between that state and Delaware not ended in Delaware's favor. But the Delaware flag defiantly flies above this strip of land, so we call it the Delaware Wedge.
How did this land dispute arise? In State and National Boundaries of the United States, Gary Alden Smith explains. In 1682, King Charles II separated the land of Penn's colony from that of what would become Delaware by a 12-mile arc extending around New Castle. This is why the northern boundary of Delaware is round. Maryland's eastern boundary was later defined with a right angle, leading to this small plot of land left unaccounted for.
Pennsylvania claimed it, arguing that Mason-Dixon Line surveyed in the 1760s allotted the land, by default, to Pennsylvania. U.S. military engineers surveying the area marked it as Pennsylvanian territory.
But the people who lived in the area identified as Delawareans, voted in that state, and paid taxes to Dover. In 1921, Congress approved of this de facto border after both state legislatures approved.
This historical marker notes the dispute that, thankfully, never boiled over into open warfare.