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.