 Llanfyllin means "church or parish" (llan) "of St Myllin" ('m' frequently mutates to 'f' in Welsh).The town lies in the valley of the River Cain near the Berwyn Mountains in Montgomeryshire, southwest of Oswestry and from Montgomery.The River Cain is joined by the small River Abel in Llanfyllin (presumably named after Cain and Abel in the Bible), and meanders through the valley, flowing into the River Vyrnwy at Llansantffraid.