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Separately, the two halves of the Twin Tiers region are known as the Southern Tier region in the state of New York and the Northern Tier region in the Commonwealth of Pennsylvania. The "Northern" and "Southern" designations are relative to the states in which they are located, not relative to each other.
The Twin Tiers region is usually defined as including these counties:
|Northern Tier||Southern Tier|
McKean, Potter, and (less often) Cameron Counties refer to themselves as part of the Twin Tiers, but almost never consider themselves part of the Northern Tier, instead going by the name "Northern Pennsylvania". Significant ambiguity often exists in regions: for instance, the western part of the region (McKean and Potter Counties) often associate themselves with St. Marys, a city larger than any city in that area, but in Elk County, just south of what is considered "Northern Tier" by any standard. This broader area is also known as the Pennsylvania Wilds.
Erie County, Pennsylvania, and Warren County, Pennsylvania, are almost never considered part of the Twin Tiers, though portions of Warren County are occasionally associated with the rest of the region.
The region was historically a disputed territory in the history of the United States prior to its founding. The Northern Tier was claimed by Pennsylvania Colony and Connecticut Colony, while the Southern Tier was claimed by Pennsylvania, Massachusetts Bay Colony, and New York Colony. Various treaties and land sales eventually placed the Northern Tier in Pennsylvania's hands and the Southern Tier in New York's.
- Knaus, Don (February 11, 2015). "The Twin Tiers Twins". Mountain Home Magazine. Mountain Home Magazine. Retrieved January 24, 2017.