A legend which is found repeatedly at different places, having the same plot in every case but with place names and/or topographical details tailored to fit the individual site, is called ‘migratory’ (e.g. Devil's Dyke, Hangman's Stone). Owing to the wide distribution of these story-patterns, both in Britain and on the Continent, it is assumed they have circulated for several centuries; some can be proved to have medieval or classical forerunners.
To facilitate identification, a numbering system was devised by Reidar Th. Christiansen on the basis of Norwegian examples (The Migratory Legends (1958)). It is used in Briggs, 1970-1, and in most professional collections since the 1960s, but needs further expansion.