A plen-an-gwarry or plain-an-gwary, is a "playing-place" or round, a medieval amphitheatre found in Cornwall. A circular outdoor space used for plays, sports, and public events, the plen-an-gwary was a Cornish variant of a construction style found across Great Britain. Formerly common across Cornwall, only two survive nearly complete today: the Plain in St Just in Penwith and Saint Piran's Round near Perranporth.

Cornish is a Southwestern Brittonic language of the Celtic language family. Along with Welsh and Breton, Cornish is descended from the Common Brittonic language spoken throughout much of Great Britain before the English language came to dominate. For centuries, until it was pushed westwards by English, it was the main language of Cornwall, maintaining close links with its sister language Breton, with which it was mutually intelligible, perhaps even as long as Cornish continued to be spoken as a vernacular. Cornish continued to function as a common community language in parts of Cornwall until the mid 18th century, and there is some evidence for traditional speakers of the language persisting into the 19th century.