Fluency [is] smooth, rapid, effortless use of language. - David Crystal

The main metaphor that is used to explain the historical relationship is that of the language family or family tree.

The growth in linguistic awareness about the problem, and the emergence of an associated activism, was one of the most exciting developments of the 1990s. Although awareness is still poor among the general public, the issues are now being much more widely discussed at professional levels, in a variety of international, national, regional, and local contexts. At one extreme, there are major campaigns such as those involved in promulgating the Barcelona Declaration of Linguistic Rights, or such initiatives as the `Red Book on Endangered Languages’ (part of the Tokyo Clearing House project). At the other extreme, there is lively debate taking place within many of the endangered communities themselves. Mechanisms and structures are now in place to channel energies.

The aim of linguistics is to go beyond the study of individual languages to determine what the universal properties of language are, and to establish a ‘universal grammar’ that would account for the range of linguistic variation that is humanly possible.