Historians generally assert that rackets began as an 18th-century pastime in London's King's Bench and Fleet debtors' prisons. The prisoners modified the game of fives by using tennis rackets to speed up the action. They played against the prison wall, sometimes at a corner to add a sidewall to the game. Rackets then became popular outside the prison, played in alleys behind pubs. It spread to schools, first using school walls, and later with proper four-wall courts being specially constructed for the game. The lithograph at right from the late 1700s shows school boys 'hitting up' outside the Harrow School 'Old School' buildings.