Comparative linguistics (originally
comparative philology) is a branch of
historical linguistics that is concerned with comparing languages in order to establish their historical relatedness. Languages may be related by convergence through borrowing or by genetic descent.
The fundamental technique of comparative linguistics is to compare phonological systems, morphological systems, syntax and the lexicon of two or more languages using a technique known as the comparative method. In principle, every difference between two related languages should be explicable to a high degree of plausibility, and systematic changes, for example in phonological or morphological systems, are expected to be highly regular (ie consistent). In practise, the comparison may be more restricted, eg just to the lexicon. In some methods it may be possible to reconstruct an earlier proto-language. Although the proto-languages reconstructed by the comparative method are hypothetical, a reconstruction may have predictive power. The most notable example of this is Saussure's proposal that the Indo-European consonant system contained laryngeals, a type of consonant attested in no Indo-European language known at the time. The hypothesis was vindicated with the discovery of Hittite, which proved to have exactly the consonants Saussure had hypothesized in the environments he had predicted.
Where languages are derived from a very distant ancestor, and are thus more distantly related, the comparative method becomes impracticable. In particular, attempting to relate two reconstructed proto-languages by the comparative method has not generally produced results that have met with wide acceptance. A number of methods based on statistical analysis of vocabulary have been developed to overcome this limitation such as lexicostatistics and mass comparison. The former uses lexical cognates like the comparative method but the latter uses only lexical similarity. The theoretical basis of such methods is that vocabulary items can be matched without a detailed language reconstruction and that comparing enough vocabulary items will negate individual inaccuracies. Thus they can be used to determine relatedness but not to determine the proto-language.
The earliest method of this type was the comparative method, which was developed over many years, culminating in the nineteenth century. This uses a long word list and detailed study. However, it has been criticized for example as being subjective.[citation needed] The comparative method uses information from two or more languages and allows reconstruction of the ancestral language. The method of Internal reconstruction uses only a single language, with comparison of word variants, to perform the same function. Internal reconstruction is more resistant to interference but usually has a limited available base of utilizable words and is able to reconstruct only certain changes (those that have left traces as morfophonological variations).
