Interest in transformational grammar has led in turn to increased interest in comparative linguistics. The differences between languages are not uniform. When languages resemble each other in a systematic way, they are said to be genetically related. Such relationships have been established in many cases, but almost always on the basis of the sounds of the languages and the way the sounds are grouped in systematic patterns. It is more difficult to compare the grammatical structures of languages. Maximal groups of related languages are called families, or stocks. A language that does not appear genetically related to any existing language is termed a language isolate.
Languages of the Indo-European and Afroasiatic families have traditionally received vastly more scholarly attention than the others. These languages actually represent a very small part of the world linguistic spectrum. As a consequence, most generalized statements about language, grammar, and related matters made before 1920 are not valid. Few authorities agree on all points of language classification and analysis, and knowledge of the languages of some isolated regions (e.g., Australia, New Guinea, and E Siberia) is still too scanty to permit proper classification.