Or the valley of the river Tywi or Towy
which rises in the Cambrian Mountains in mid Wales before making a short sixty mile dash south-east through Llandovery and and Llandeilo and reaching the sea at the town of Carmarthern.
Never a medieval kingdom in its own right, Ystrad Tywi remained a valuable territory that was fought over down the centuries; at various times being under the control of the various kings of Dyfed, Seisyllwg, Deheubarth and Gwynedd, and in the end, of course, the Normans.
The river itself marked the principle divisions of the territory between Cantref Mawr (the Greater Hundred) on the north bank, and Cantref Bychan (the Lesser Hundred) as well as Cantref Cydweli (the Hundred of Kidwelli) on the south bank.
Roughly speaking equivalent to modern Carmarthenshire.