Most languages are grouped into families. A properly defined family should be a genetic unit, which means that all its members should derive from a common ancestor. The ancestor is very seldom known to us directly, since most languages have a very short recorded history. However, it is possible to recover many of the features of the common ancestor of related languages by applying comparative method -- a reconstructive procedure worked out by nineteenth-century linguists. In this way one can demonstrate the family status of many of the groupings listed below.
Language families can be subdivided into smaller units, conventionally referred to as "branches" (because the history of a language family is often represented as a "family tree" diagram).
The common ancestor of a family (or branch) is known as its "protolanguage". For example, the reconstructible protolanguage of the well-known Indo-European family is called Proto-Indo-European" (not known from written records, since it was spoken before the invention of writing). Sometimes a protolanguage can be identified with a historically known language. Thus, provincial dialects of Latin ("Vulgar Latin") gave rise to the modern Romance languages, so the Proto-Romance language is more or less identical with Latin (if not exactly with the literary Latin of the Classical writers).
There are also constructed languages.