Ystrad Tywi
Land in medieval Wales was divided into "cantrefi", which were themselves divided into smaller "cymydau" (commotes). The word "cantref" is derived from "cant" ("a hundred") and "tref" ("town" in modern Welsh, but formerly used for much smaller settlements). The "cantref" is thought to be the original unit, with the commotes being a later division. "Cantrefi" could vary considerably in size: most were divided into two or three commotes, but the largest, the "Cantref Mawr" (or "Great Cantref") in Ystrad Tywi (now in Carmarthenshire) was divided into seven commotes. To give an idea of the size of a "cantref", the island of Anglesey was divided into three "cantrefi": Cemais, Aberffraw and Rhosyr.