The Kreuznach document mentions possessions of Gerulf in and around Leeuwarden and between Vlie and Lonbach.
However, since the formation of the Zuiderzee and obstruction from the Afsluitdijk, the IJssel no longer flows into its estuary, the Vlie, now restricted in meaning and referring only to the strait separating the islands of Vlieland (itself named after the Vlie strait) and Terschelling.