Wikipedia begins the description of “County Corporate” with –
A county corporate or corporate county was a type of sub-national division used for local government in England, Ireland, and Wales.
Counties corporate were created during the Middle Ages, and were effectively small self-governing counties. They usually covered towns or cities which were deemed to be important enough to be independent from their counties and a county corporate could also be known as a county of itself.
While they were administratively distinct counties, with their own sheriffs and lieutenancies, most of the counties corporate remained part of the “county at large” for purposes such as the county assize courts. From the 17th century the separate jurisdictions of the counties corporate were increasingly merged with that of the surrounding county, so that by the late 19th century the title was mostly a ceremonial one.
To read the full article, a full list of the original Cities & Towns with Sheriffs and many references, please click here