Throughout this book, we have been talking about languages such as English, Spanish or Swahili as if there was a single variety of each in everyday use. That is, we have largely ignored the fact that every language has a lot of variation, especially in the way it is spoken. If we just look at English, we find widespread variation in the way it is spoken in different countries such as Australia, Britain and the USA. We can also find a range of varieties in different parts of those countries, with Lee Tonouchi's account of ‘Trick-O-Treat’ in Hawai'i as just one example. In this chapter, we investigate aspects of language variation based on where that language is used, as a way of doing linguistic geography.