This interplay between the real and the virtual provides a starting point for the development of our taxonomy. A direct result of the need to recognise this coupling is that many of the categories we will consider for taxonomising and understanding mobile and context-aware computation have counterparts in both the real physical world and virtual electronic world. There are important differences and the virtual world does not always behave in ways we have come to expect from the physical world and these differences are often exploited by designers and developers.