Llannerch (sometimes spelled Llanerch) was a commote in the cantref of Dyffryn Clwyd which later became the Marcher Lordship of Ruthin. Situated in an area south of Ruthin the commote covered an area of 9,000 acres (14 sq mi; 3,600 ha) which included the parishes of Llanfair Dyffryn Clwyd and Llanelidan and 19 townships. Much of the land within the commote was owned by the Bishopric of Bangor.