The spatial dimension of labour relations has been a constant, if undertheorized, dimension of study since the nineteenth century. The concept of national systems of labour relations is widely used, using indicators such as national legislative regimes, national organizational bodies and traditions, and elements of national culture to define the particular form taken. The idea of international labour relations arose in the nineteenth century in contexts such as the international trade secretariats, international labour unions and mobilization and, later, in the International Labor Organization (post-1919). There has also been a traditional interest in regional labour relations, either at sub-national level (for example, the Appalachian mining industry, the Lancashire cotton industry), or at the supra-national level (for example, across the US-Mexican border, within the Association of South East Asian Nations (ASEAN) economies). Some theoretical approaches in labour relations have combined spatial concerns with industrial characteristics, as in the isolated mass analysis of labour behaviour. Finally, the intersection of labour relations theory and meta-theory has also provided a spatial understanding of the institutions and process of labour relations, as in, for example, the approach of dependency theory to development and underdevelopment and its understanding of the role of labour relations in the process of unequal exchange.