Grimm's law was the first discovery of a systematic sound change, and it led to the creation of historical phonology as a separate discipline of historical linguistics. The correspondence between Latin p and Germanic f was first noted by Friedrich von Schlegel in 1806. In 1818, Rasmus Rask extended the correspondences to other Indo-European languages such as Sanskrit and Greek, and to the full range of consonants involved. In 1822, Jacob Grimm put forth the rule in his book Deutsche Grammatik and extended it to include standard German. He noticed that there were many words which had different consonants from what his law predicted, and these exceptions defied linguists for a few decades, but they eventually received explanation from Danish linguist Karl Verner in the form of Verner's law.