The formula for the border as ‘barrier and sieve’ does not come from a theory of architecture. I found it in a recent book, written by an American medical expert, on the functions of the skin. After I had studied this doctor’s reflections on skin as border, they continued to haunt my thoughts even as I began to think about the function of political borders and other spatial divisions in architecture. With this in mind I should like to begin with a speculative thought experiment using the skin as a model of the border – of national border and regional border as well as the constructed border surfaces in architecture.