The regions, with their names and numbering are taken from The Relative Hills of Britain by Alan Dawson.
A Corbett is "a Scottish hill between 2500 and 2999 feet high with a drop of at least 500 feet (152.4m) on all sides". They are named after the list's original compiler, J.Rooke Corbett.
The Corbetts are a subset of the Marilyns.
(Corbetts appear as Class=C on this website).