The notion of locating the birth of the social movement in a specific time and place occurred to Tilly while he was writing his pathbreaking work on the transformation of British popular politics, "Popular Contention in Great Britain, 1758–1834". In the process of collating and analyzing the 8,088 contentious gatherings (CGs) that form the basis of the book, Tilly noticed that one momentous change in how ordinary people made collective claims on public authorities turned out to be the invention of what we now call the social movement. "I couldn't help seeing that in Britain, at least, social movements didn't exist in the mid-18th century but had become a dominant form of popular politics by the 1830s. That started me writing about the history of social movements, first in Western Europe, then finally across the world as a whole."