What is the rlationship between the Grangerfords and Shepherdsons?

While out hunting one day, Huck and Buck hear a horse approaching behind them. Quickly, they run behind a bush and wait to see who arrives. Harvey Shepherdson passes by and Buck takes a shot at him, knocking off his hat. Harvey then follows the two boys into the woods but is unable to catch them. At this point, Buck explains the family feud to Huck. For over thirty years, the men in each family have been committed to killing off the men in the opposing family. No one remembers why the feud started, but several men have been killed each year.