LANGUAGE FAMILY. A group of languages which are assumed to have arisen from a single source: ENGLISH, FRENCH, GERMAN, GREEK, PERSIAN, RUSSIAN, SANSKRIT, and WELSH are all members of the INDO-EUROPEAN language family, and are considered to have descended from a common ancestor. Common ancestry is established by finding systematic correspondences between languages: English repeatedly has /f/ where Latin has /p/ in words with similar meaning, as in father/pater, fish/piscis, flow/pluo rain. It also often has /s/ where Greek has /h/, as in six/héx, seven/heptá, serpent/hérpein to creep. In addition, English and German compare adjectives in similar ways, as in rich, richer, richest: reich, reicher, reichste. These and other correspondences indicate that the languages are cognate (genetically related). Various related words can be compared in order to reconstruct sections of a hypothetical ancestor language. The process of comparison and reconstruction is traditionally known as comparative PHILOLOGY, more recently as comparative historical linguistics. This process formed the backbone of 19c language study, though in the 20c it has become one branch among many. A ‘family tree’ diagram (not unlike a genealogy) is commonly used to represent the relationships between the members of a linguistic family, in which an initial parent language ‘gives birth’ to a number of ‘daughters’, which in turn give birth to others. This can be useful, but is rarely an accurate representation of how languages develop, since it suggests clean cuts between ‘generations’ and between ‘sister’ languages, and implies that languages always become more divergent. In fact, languages generally change gradually, and there is often considerable intermixing among those which remain geographically adjacent. See LANGUAGE CHANGE, LINGUISTIC TYPOLOGY.