Arthur J. Penty, an English architect in private practice in York at the turn of the century, became associated with Barry Parker and Raymond Unwin as a freelance designer and exerted a fundamentally important (though largely unsung) influence on the stylistic principles now associated with Parker and Unwin's work at the First Garden City, Letchworth (founded 1903) and at Hampstead Garden Suburb in London (commenced 1905). He was a competent Arts and Crafts designer during a late phase of this idiom's effectiveness in England, believing it to be both culturally and socially appropriate in its reflection of the English temperament and its demand for high quality production. His concerns for the latter prompted him to be an architectural theorist, to popularize the work of Voysey and Lethaby, and to advocate greater on-site collaboration between architects and craftsmen and the virtual abolition of designing on paper. It also persuaded him to become a political activist and to originate a movement--Guild Socialism-- which placed great faith in the potential governance of education and production by restored crafts guilds and which enjoyed a brief moment of success in the form of a National Guilds League just after the First World War. Medievalism is the key concept linking all aspects of his life's work--his devotion to the teachings of Morris, his respect for likeminded 19th-century practical idealists, his wish to encourage a return to systems of quality, control and production effective in the Middle Ages, and his "medievalist" detailing of several of Parker and Unwin's landmark buildings. [ABSTRACT FROM AUTHOR]