 It concerns the correspondence of consonants between the ancestral Proto-Indo-European language and its Germanic descendants, Low Saxon and High German, and was first fully stated by Grimm in the second edition of the first part of his "Grammar".The correspondence of single consonants had been more or less clearly recognized by several of his predecessors, including Friedrich von Schlegel, Rasmus Christian Rask and Johan Ihre, the last having established a considerable number of "literarum permutationes", such as b for f, with the examples "bœra" = "ferre" ("to bear"), "befwer" = "fibra" ("fiber").Rask, in his essay on the origin of the Icelandic language, gave the same comparisons, with a few additions and corrections, and even the same examples in most cases.