“A cantref is a measurement of a hundred (literally, it means “one hundred”). A commot is a community, the word ultimately deriving from the same root as Cymru–comrad, compatriot, neighbor.” The list of cantrefs and commotes from the Red Book of Hergest is found here: http://www.maryjones.us/ctexts/cantref.html
In the Middle Ages, Gwynedd had and fifteen cantrefs and thirty-six commotes. Overall, it was the largest of the regions of Wales.
“The antiquity of the cantrefi is demonstrated by the fact that they often mark the boundary between dialects. Some were originally kingdoms in their own right, others may have been artificial units created later. (Davies, John; Nigel Jenkins, Menna Baines and Peredur I. Lynch (2008), The Welsh Academy Encyclopedia of Wales, Cardiff: University of Wales Press, pp. 113)
“Cantrefi were of particular importance in the administration of the Welsh law. Each cantref had its own court, which was an assembly of the “uchelwyr“, the main landowners of the cantref. This would be presided over by the king if he happened to be present in the cantref, or if he was not present by his representative. Apart from the judges there would be a clerk, an usher and sometimes two professional pleaders. The cantref court dealt with crimes, the determination of boundaries and matters concerning inheritance. The commote court later took over many of the functions of the cantref court, and in some areas the names of the commotes are much better known than the name of the cantref of which they formed parts.” http://en.wikipedia.org/wiki/Cantref