In Scotland they are largely based around the 1975 - 1996 districts, whereas in Wales they are based on districts (the second-tier administrative divisions which functioned between 1974 and 1996) or counties. The individual AOSs are between 400 km2 and 4,000 km2 in size.
In Scotland and Wales, the administrative areas are based on districts or counties, with the size of the administrative units ranging from 400 square kilometers to 4,000 square kilometers.