In linguistics, the comparative method is a technique for studying the development of languages. It requires the use of two or more languages. It is opposed to the method of internal reconstruction, which studies the internal development of a single language over time. Ordinarily both methods are used together. They constitute a powerful means to reconstruct prehistoric phases of languages, to fill in gaps in the historical record of a language, to study the development of phonological, morphological, and other linguistic systems, and to confirm or refute hypothesized relationships between languages.
The comparative method was gradually developed during the course of the 19th century. The key contributions were made by the Danish scholars Rasmus Rask and Karl Verner and the German scholar Jacob Grimm. The first linguist to offer reconstructed forms from a proto-language was August Schleicher, in his Compendium der vergleichenden Grammatik der indogermanischen Sprachen, originally published in 1861.
Contrary to what is often assumed today, Schleicher and the other comparative linguists of the 19th century did not view the comparative method as a means to establish the validity of the language families they studied, which they considered to be already established through an interlocking web of lexical and morphological similarities. Characteristic is Schleicher’s explanation of why he took the then-radical step of offering reconstructed forms:
In the present work an attempt is made to set forth the inferred Indo-European original language side by side with its really existent derived languages. Besides the advantages offered by such a plan, in setting immediately before the eyes of the student the final results of the investigation in a more concrete form, and thereby rendering easier his insight into the nature of particular Indo-European languages, there is, I think, another of no less importance gained by it, namely that it shows the baselessness of the assumption that the non-Indian Indo-European languages were derived from Old-Indian (Sanskrit).
During the first half of the 20th century, the conviction gradually took hold that reconstructions arrived at through the comparative method were the only valid means to establish genetic (i.e. genealogical) relationship between languages. This has since remained the prevailing view among historical linguists. It is contested only by followers of Joseph Greenberg.
Although the following article deals only with the role of the comparative method in demonstrating genetic relationship, it is important to realize that this is only one application of the comparative method, which has rightly been described as the central tool of historical linguistics. For example, André Martinet uses the comparative method in his influential Economie des changements phonétiques (2005/1955) to study the evolution of sound systems over time and, via this, to develop generalizations about the nature of sound systems as synchronic entities.