description of the regular correspondences in Indo-European languages formulated by Jacob Grimm in his Deutsche Grammatik (1819–37; “Germanic Grammar”); it pointed out prominent correlations between the Germanic and other Indo-European languages of Europe and western Asia. The law was a systematic and coherent formulation, well supported by examples, of patterns recognized as early as 1814 by the Danish philologist Rasmus Kristian Rask. It is important for historical linguistics because it clearly demonstrates the principle that sound change is a regular phenomenon and not a random process affecting only some words, as had been thought previously.

Grimm described two consonant shifts involving essentially nine consonants. One shift (probably a few centuries before the Christian era) affected the Indo-European consonants and is evident in English, Dutch, other Low German languages, and Old Norse. The other shift (about the 6th century AD) was less radical in scope and affected the Germanic consonants, resulting in the consonant system evident in Old High German and its descendants, Middle High German and Modern High German (standard German). According to the law, the ancient unvoiced p, t, k became the English unvoiced f, th, h and the Old High German f, d, h, producing such correlations as that between the initial consonants of Greek pod-, English fod, and Old High German fuo. The law further stated that the ancient voiced b, d, g became the English unvoiced p, t, k and the Old High German spirant stops f, ts, kh; hence, the correlation between Latin duo, English “two,” and modern German zwei (pronounced “tsvai”). Also, the originally voiced bh, dh, gh became the English voiced b, d, g and the Old High German p, t, k; compare Sanskrit bhárati, English “bear,” and the Upper German dialects of Old High German ki-peran (later standard German ge-bären). The Old High German examples show the second shift in addition to the first, which is seen in English.