Whenever I imagine a frontier, I imagine the borderline say between France and Belgium. Everyone knows where it is, there are checkpoints and border guards, there is the administration of the state and 1 cm that way, you are in the land of croissants and Voltaire, a cm the other way and its Tintin and Hercule Poirot. Of course that is not how frontiers actually work. In the United States one of the fundemental points in John Elliot's recent study is that in both British and Spanish empires the frontier was not a place but a zone. The zone might fluctuate- it might extend- in the British case normally through trade or colonisation, in the Spanish through the movement of missionaries particularly those associated with the great orders- the Franciscans, the Dominicans etc- or it might contract- as the Spanish or British abandoned places too expensive to defend and too exposed. There were pressures outwards like population pressure in the Eastern US seaboard, or pressures to contract like the Portugeese who moved on Spanish allied Indians in the Amazonian rainforest to recruit slaves for the plantations in India. Whatever it was though, we are incorrect to ever feel that the frontier was here and moved westwards or eastwards forever until European colonisation was complete: rather than being a neat line on a map, it was a zone, a status of uncertainty which permeated American life with the fear of conquest and the exhileration of religious expansion or agricultural opportunity.