So far, we have discussed how languages evolve from earlier forms through changes in pronunciation, vocabulary and grammar. As a result of this evolution, languages that share a common ancestor can be classified into groupings of various sizes, depending on how long ago that common ancestor was spoken. For example, Proto-West-Germanic, the common ancestor of West Germanic languages such as English, Dutch and German, was spoken more recently than Proto-Germanic, the common ancestor of those West Germanic languages, as well as North Germanic languages such as Icelandic, Norwegian, Swedish and Danish. In turn, Proto-Germanic was spoken more recently than Proto-Indo-European, the common ancestor of an even larger grouping, the Indo-European language family.

One important factor in the evolution of languages is language contact. As has been discussed in Section 5.3 and elsewhere throughout this book, language contact and the resulting lexical and grammatical borrowing may reshape the language to be quite different from its relatives. While all of the world's languages have at some time been influenced by some language or other, in this chapter we will discuss very specific types of languages that arise out of a particularly close contact between groups of people who do not speak each other's languages, but who for some reason or another simply must work out a means of communicating with each other. These languages are known as pidgins and creoles.