:Ystlyg

{{Short description|Welsh medieval cantref}}

File:Wales.medieval.cymydau.jpg

Ystlyg ({{langx|en|possibly curve{{cite book|last1=Jones|first1=John|title=The History of Wales|date=1824|publisher=J Williams|location=London|page=105}} or open country{{cite book|last1=Cathrall|first1=William|title=The History of North Wales Vol II|date=1828|location=Manchester|page=342}}}}) 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 (cymydau) of Deuddwr in the north,{{cite book|last1=Carlisle|first1=Nicholas|title=A topographical dictionary of the Dominion of Wales (see Llandrinio)|date=1811|publisher=Society of Antiquaries|location=London|url=https://archive.org/details/walestopographic00carluoft}} Ystrad Marchell in the centre, Llannerch Hudol (also written Llannerchwdwl, Llanerchydol or Llannerchudol) in the south, and Y Gorddwr (or Corddwr{{cite book|last1=Carlisle|first1=Nicholas|title=A topographical dictionary of the Dominion of Wales (see Garth)|date=1811|publisher=Society of Antiquaries|location=London|url=https://archive.org/details/walestopographic00carluoft}}) in the east beyond the River Severn and Offa's Dyke.{{cite book|last1=Rees|first1=William|title=An Historical Atlas of Wales from Early to Modern Times|date=1951|publisher=Faber & Faber}}

Ystad Marchell, Llannerch Hudol and Deuddwr formed the Teirswydd (English: three [commotes] (literally, swydd is post as in an office)) 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.{{cite book|last1=Smith|first1=J Beverley|title=Llywelyn ap Gruffudd: Prince of Wales|date=2014|publisher=University of Wales Press|page=370}} Y Gorddwr had remained under the control of the Corbet family (barons of Caus) and therefore the English king.{{cite book|last1=Smith|first1=J Beverley|title=Llywelyn Ap Gruffudd: Prince of Wales|date=2014|publisher=University of Wales Press|page=416}}

The cantref of Ystlyg (excluding Y Gorddwr) corresponds to the later hundred of Deuddwr.