Penllyn (head of the lake i.e. Bala Lake or Llyn Tegid) was a medieval cantref originally in the Kingdom of Powys but annexed to the Kingdom of Gwynedd. It consisted of the commotes (cymydau) of Edeyrnion, Dinmael, Penllyn is Treweryn and Penllyn uwch Treweryn (is signifying 'below' and uwch 'above' the River Tryweryn). On the north and west it bordered Gwynedd (the cantrefi of Tegeingl, Rhufoniog, Dunoding and Meirionydd); it bordered the Powys cantrefi of Maelor, Mochnant and Cyfeiliog on the east and south.