One of the most ingenious revivals of ancient laws was the so-called "distraint of knighthood." According to an unrepealed statute of Edward, all landholders with a stipulated income were required to become knights. In the days of Edward the law had been designed to provide a properly equipped force of armed horsemen. Now, when 40-pound incomes had become far more common, it was used to mulct a very large group. Men had to pay a heavy fee if they became knights, and they were fined if they refused.
(from A History of England and the British Empire)