The families lived on opposite sides of a stream called Tug Fork in the Appalachian Mountains.
The McCoys resided in Pike County, Kentucky, and the Hatfields in Logan County, West Virginia.
How the feud got started is not known, but it got under way in earnest with the killing of a Hatfield in 1882.
The fighting went on intermittently into the 1890s and was not completely over until after 1910.