We’ve seen how as we extend the boundaries of speech communities, we get fewer and fewer shared conventions. When we reach the level of a language such as English, Spanish, or Mandarin Chinese, we have a speech community which shares a set of conventions (in some cases a standard dialect) which allows people in the community to communicate with one another despite dialect differences. But we can go beyond a language. So for English, we could extend the boundaries to include the Netherlands, Germany, Scandinavia, and some other regions in western Europe. We’d now find a much smaller set of shared conventions. All speakers in this large “community”, for example, share a word meaning ‘all’ which is similar in pronunciation to the English word “all”. But there would be no reason to call this set of conventions a “language” since the speakers obviously do not understand each other and do not belong to a single political unit with a single standard dialect. Instead we refer to this set of conventions, or set of languages, as a “language family”. In this case, the Germanic languages. The members of a language family resemble each other because they are genetically related; that is, historically they derived from a common ancestor language. (Note that this use of the word genetic differs somewhat from its use in biology; the speakers of Germanic languages are not necessarily genetically closer to one another than they are to the speakers of other languages.) The ancestor of the modern Germanic languages was not a written language, so we can only infer what it was like.
In most cases we can go even further back; the ancestor languages of two or more families themselves may have had a common ancestor language. Thus the modern Romance languages, including Spanish, French, Italian, Portuguese, Catalan, and Romanian; the modern Germanic languages; and many other languages spoken today in Europe, the Middle East, and South Asia, apparently descended from a much older (and also unwritten) language. This means we can group all of these languages into a single family, in this case the one we call Indo-European. Sometimes, to distinguish the lower from the higher levels within a family tree of languages, we use “language family” only for the largest grouping (for example, Indo-European) and “branch” to refer to groupings within this (for example, Germanic and Romance). Note that there may be many intermediate levels in the family tree of languages. Within Germanic, for example, there is North, including the Scandinavian languages, and West, including English, Dutch, and German.
Note also that languages may resemble each other in one way or another for reasons other than a genetic relationship. The main non-genetic source of similarity is language contact; when the speech communities for two language are in close cultural contact, their languages often influence one another. So modern Japanese vocabulary includes thousands of words borrowed from Chinese and uses the Chinese writing system (as well as writing systems specific to Japanese). But, except in the sense that all human languages may be ultimately related to one another, there is no evidence that Japanese is genetically related to Chinese. A more complicated situation occurred in Western Asia with the complicated cultural influences among people speaking Arabic, Persian, and Turkish. These three languages belong to separate language families (Afro-Asiatic, Indo-European, and Altaic, respectively), which are either unrelated to one another or only very distantly related, but Turkish and Persian have borrowed many words from Arabic, Turkish has also borrowed many words from Persian, and Persian borrowed its writing system from Arabic.