The history of Monopoly can be traced back to 1903, when American Elizabeth (Lizzie) J. Magie Phillips created a game through which she hoped to be able to explain the single tax theory of Henry George. It was intended as an educational tool to illustrate the negative aspects of concentrating land in private monopolies. Magie took out a patent in 1904. Her game, The Landlord's Game, was self-published, beginning in 1906. A series of variant board games based on her concept was developed from 1906 through the 1930s that involved the buying and selling of land and the development of that land.