Why we might want to know how similar or different varieties are to each other

What are the historical origins of Standard English, and how might we find out? Are regional varieties of English getting more similar to each other as a result of increased geographical and social mobility? Is it true that varieties of English spoken in former Celtic-speaking areas such as Cornwall, Wales, and the Highlands of Scotland are more similar to Standard English than other varieties? How similar are Scots and English, and are they more or less different than, for example, German and Dutch?

These, and many others like them, are questions which we, as linguists and dialectologists, would like to be able to answer. They all involve determining the relationships between varieties of a language (or indeed between different languages) in a way which goes beyond looking at individual features. In determining and even measuring the similarities and differences between dialects, we can begin to answer questions about their status and history, and the connections between these and the society, history, and geography of the people who speak them. This chapter is about how we determine the degree of relatedness between linguistic varieties – of course, varieties of the same language are related, and are similar, so we need to get beyond yes–no questions and instead figure out how close the relationships between them are. In §5.2, we examine traditional approaches to the issue, specifically the use of isoglosses for determining dialect boundaries.