Structure of a family
Language families can be divided into smaller phylogenetic units, conventionally referred to as branches of the family because the history of a language family is often represented as a tree diagram. A family is a monophyletic unit; all its members derive from a common ancestor, and all attested descendants of that ancestor are included in the family. (Thus, the term family is analogous to the biological term clade.)
Some taxonomists restrict the term family to a certain level, but there is little consensus in how to do so. Those who affix such labels also subdivide branches into groups, and groups into complexes. A top-level (i.e., the largest) family is often called a phylum or stock. The closer the branches are to each other, the closer the languages will be related. This means if a branch off of a proto-language is 4 branches down and there is also a sister language to that fourth branch, then the two sister languages are more closely related to each other than to that common ancestral proto-language.
The term macrofamily or superfamily is sometimes applied to proposed groupings of language families whose status as phylogenetic units is generally considered to be unsubstantiated by accepted historical linguistic methods. For example, the Celtic, Germanic, Slavic, Italic, and Indo-Iranian language families are branches of a larger Indo-European language family. There is a remarkably similar pattern shown by the linguistic tree and the genetic tree of human ancestry
that was verified statistically. Languages interpreted in terms of the putative phylogenetic tree of human languages are transmitted to a great extent vertically (by ancestry) as opposed to horizontally (by spatial diffusion).
Some closely knit language families, and many branches within larger families, take the form of dialect continua in which there are no clear-cut borders that make it possible to unequivocally identify, define, or count individual languages within the family. However, when the differences between the speech of different regions at the extremes of the continuum are so great that there is no mutual intelligibility between them, as occurs in Arabic, the continuum cannot meaningfully be seen as a single language.
A speech variety may also be considered either a language or a dialect depending on social or political considerations. Thus, different sources, especially over time, can give wildly different numbers of languages within a certain family. Classifications of the Japonic family, for example, range from one language (a language isolate with dialects) to nearly twenty—until the classification of Ryukyuan as separate languages within a Japonic language family rather than dialects of Japanese, the Japanese language itself was considered a language isolate and therefore the only language in its family.
Most of the world's languages are known to be related to others. Those that have no known relatives (or for which family relationships are only tentatively proposed) are called language isolates, essentially language families consisting of a single language. An example is Basque. In general, it is assumed that language isolates have relatives or had relatives at some point in their history but at a time depth too great for linguistic comparison to recover them.
A language isolated in its own branch within a family, such as Albanian and Armenian within Indo-European, is often also called an isolate, but the meaning of the word "isolate" in such cases is usually clarified with a modifier. For instance, Albanian and Armenian may be referred to as an "Indo-European isolate". By contrast, so far as is known, the Basque language is an absolute isolate: it has not been shown to be related to any other language despite numerous attempts. Another well-known isolate is Mapudungun, the Mapuche language from the Araucanían language family in Chile. A language may be said to be an isolate currently but not historically if related but now extinct relatives are attested. The Aquitanian language, spoken in Roman times, may have been an ancestor of Basque, but it could also have been a sister language to the ancestor of Basque. In the latter case, Basque and Aquitanian would form a small family together. (Ancestors are not considered to be distinct members of a family.)
A proto-language can be thought of as a mother language (not to be confused with a mother tongue, which is one that a specific person has been exposed to from birth), being the root which all languages in the family stem from. The common ancestor of a language family is seldom known directly since most languages have a relatively short recorded history. However, it is possible to recover many features of a proto-language by applying the comparative method, a reconstructive procedure worked out by 19th century linguist August Schleicher. This can demonstrate the validity of many of the proposed families in the list of language families. For example, the reconstructible common ancestor of the Indo-European language family is called Proto-Indo-European. Proto-Indo-European is not attested by written records and so is conjectured to have been spoken before the invention of writing.