Истрад-Тиви
Dyfed may have originally occupied the area that bordered the rivers Teifi, Gwili and Tywi, and included contemporary Pembrokeshire, the western part of contemporary Carmarthenshire, and with the town of Carmarthen. Dyfed eventually comprised at least seven cantrefi: Cemais, Deugleddyf, Emlyn, Cantref Gwarthaf, Pebidiog, Penfro and Rhos, with an approximate area of about . During times of strength, the kingdom expanded to additionally cover the Ystrad Tywi ("Valley of the Tywi"), including Cydweli and Gwyr, and even bordered Brycheiniog. Dyfed lost the Ystrad Tywi region to Ceredigion, another petty kingdom, in the late 7th century.