Ystlyg
Ystlyg was a medieval cantref in the Kingdom of Powys. It lay at the east of the kingdom on the border with England. It consisted of the commotes of Deuddwr in the north, Ystrad Marchell in the centre, Llannerch Hudol in the south, and Y Gorddwr in the east beyond the River Severn and Offa's Dyke.
Ystad Marchell, Llannerch Hudol and Deuddwr formed the Teirswydd which were among the lands restored into the possession of Gruffydd ap Gwenwynwyn in return for his homage and fealty by Llywelyn ap Gruffudd at Ystumanner in 1263. Y Gorddwr had remained under the control of the Corbet family and therefore the English king.
The cantref of Ystlyg corresponds to the later hundred of Deuddwr.
