A fen is an alkaline wetland, mostly found near the eastern shore of England, in and near Norfolk. According to the Sorting Hat's song in 1994, Salazar Slytherin came from fen, suggesting that he was most likely from this area.

Then again, Ireland is also known for several fen-land areas; therefore, some have suggested Slytherin might be from Ireland, as that would represent all four kingoms Hogwarts students come from.