Cefn-coed-y-cymmer is a small community on the northwestern edge of Merthyr Tydfil County Borough in Wales. It is situated in the neck of land between the rivers Taf Fawr and Taf Fechan at their confluence. The village lies within the community of Vaynor. Immediately to the north of the village is the hill of Cefn Cil Sanws on the southern slopes of which is Merthyr Tydfil Golf Club. The village is bounded both to the north and the west by the Brecon Beacons National Park.

Merthyr Tydfil is the main town in Merthyr Tydfil County Borough, Wales, administered by Merthyr Tydfil County Borough Council. It is about 23 miles (37 km) north of Cardiff. Often called just Merthyr, it is said to be named after Tydfil, daughter of King Brychan of Brycheiniog, who according to legend was slain at Merthyr by pagans about 480 CE. Merthyr generally means "martyr" in modern Welsh, but here closer to the Latin martyrium: a place of worship built over a martyr's relics. Similar place names in south Wales are Merthyr Cynog, Merthyr Dyfan and Merthyr Mawr.