The regions, with their names and numbering are taken from The Relative Hills of Britain by Alan Dawson.
A Dewey is "a mountain or hill in England, Wales or the Isle of Man, that exceeds 500 metres in height, but is below 610 metres (2000ft) in altitude; it must be separated from adjacent tops by a height difference of at least 30 metres on all sides". The definition is similar to the Hewitts and Sims, except that Hewitts must be above 2000ft, and Sims above 600m.
The Deweys are named after the list's compiler, Michael Dewey, the list appearing as one of the chapters in his book, Mountain Tables.
(Deweys appear as Class=5 (short for 500m tops) on this website, and subDeweys as class=s5).
The hills between 490 and 499m with 30m drop are also included on this site, and appear as class=4. The hills between 490 and 499m with 20-29m drop are also included as class=s4.