What is "comparative method"

n. (context linguistics English) A technique for studying the development of languages by performing a feature-by-feature comparison of two or more languages with a common ancestor.

In linguistics, the comparative method is a technique for studying the development of languages by performing a feature-by-feature comparison of two or more languages with common descent from a shared ancestor, as opposed to the method of internal reconstruction, which analyses the internal development of a single language over time. Ordinarily both methods are used together to reconstruct prehistoric phases of languages, to fill in gaps in the historical record of a language, to discover the development of phonological, morphological, and other linguistic systems, and to confirm or refute hypothesized relationships between languages.

The comparative method was developed over the 19th century. 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. Here is Schleicher’s explanation of why he offered 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).