In 1059, the right of electing the pope was reserved to the principal clergy of Rome and the bishops of the seven suburbicarian sees. In the 12th century the practice of appointing ecclesiastics from outside Rome as cardinals began, with each of them assigned a church in Rome as his titular church or linked with one of the suburbicarian dioceses, while still being incardinated in a diocese other than that of Rome.[citation needed] 

The term cardinal at one time applied to any priest permanently assigned or incardinated to a church, or specifically to the senior priest of an important church, based on the Latin cardo (hinge), meaning "principal" or "chief". The term was applied in this sense as early as the ninth century to the priests of the tituli (parishes) of the diocese of Rome. The Church of England retains an instance of this origin of the title, which is held by the two senior members of the College of Minor Canons of St Paul's Cathedral.
WHen did the rule change?
the 12th century