The comparative method developed out of attempts to reconstruct the proto-language mentioned by Jones, which he did not name, but subsequent linguists named Proto-Indo-European (PIE). The first professional comparison between the Indo-European languages known then was made by the German linguist Franz Bopp in 1816. Though he did not attempt a reconstruction, he demonstrated that Greek, Latin and Sanskrit shared a common structure and a common lexicon.[12] Friedrich Schlegel in 1808 first stated the importance of using the eldest possible form of a language when trying to prove its relationships;[13] in 1818, Rasmus Christian Rask developed the principle of regular sound changes to explain his observations of similarities between individual words in the Germanic languages and their cognates in Greek and Latin.[14] Jacob Grimm - better known for his Fairy Tales - in Deutsche Grammatik (published 1819-37 in four volumes) made use of the comparative method in attempting to show the development of the Germanic languages from a common origin, the first systematic study of diachronic language change.[15]
Both Rask and Grimm were unable to explain apparent exceptions to the sound laws that they had discovered. Although Hermann Grassmann explained one of these anomalies with the publication of Grassmann's law in 1862,[16] it was Karl Verner who in 1875 made a methodological breakthrough when he identified a pattern now known as Verner's law, the first sound law based on comparative evidence showing that a phonological change in one phoneme could depend on other factors within the same word, such as the neighbouring phonemes and the position of the accent,[17] now called conditioning environments.
Similar discoveries made by the Junggrammatiker (usually translated as
Neogrammarians) at the University of Leipzig in the late 1800s led them to conclude that
all sound changes were ultimately regular, resulting in the famous statement by Karl Brugmann and Hermann Osthoff in 1878 that
"sound laws have no exceptions".[18] This idea is fundamental to the modern comparative method, since the method
necessarily assumes regular correspondences between sounds in related languages, and consequently regular sound changes from the proto-language. This Neogrammarian Hypothesis led to application of the comparative method to reconstruct Proto-Indo-European, with Indo-European being at that time by far the most well-studied language family. Linguists working with other families soon followed suit, and the comparative method quickly became the established method for uncovering linguistic relationships.[10]