Approximately half of the language families indigenous to North America North of Mexico have been identified as isolates, families consisting of a single language. The figure is necessarily approximate for several reasons. There is an important distinction between isolates, that is, languages with no known relatives, and unclassifiable languages, those for which it is difficult or impossible to identify relatives due to the quality and quantity of documentation. A second challenge for the identification of isolates is the continuum between dialects and languages. Mutual intelligibility is of course a matter of degree and experience. Related languages are traditionally defined as those descended from a common ancestor. But a relationship cannot be simply proven or disproven. A number of languages were spoken at contact in what is now Texas and adjacent Mexico. Isolate status depends entirely on the existence of related but distinct languages.