Grimm's law, also known as the First Germanic Sound Shift, is a set of sound laws describing the Proto-Indo-European (PIE) stop consonants as they developed in Proto-Germanic in the first millennium BC, first discovered by Rasmus Rask but systematically put forward by Jacob Grimm.
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.
Grimm's law consists of three parts, forming consecutive phases in the sense of a chain shift.
The exact details of the shift are unknown, and it may have progressed in a variety of ways before arriving at the final situation.
This variety of Grimm's law is often suggested in the context of Proto-Indo-European glottalic theory, which is followed by a minority of linguists.
This theoretical framework assumes that PIE "voiced stops" were actually voiceless to begin with, so that the second phase did not actually exist as such, or was not actually devoicing but was losing some other articulatory feature like glottalization or ejectiveness.
The early Germanic *gw that had arisen from Proto-Indo-European *gʷʰ (and from *kʷ through Verner's law) further changed with various sorts: Perhaps the usual reflex was *b (as suggested by the connection of bid < *bidjaną and Old Irish guidid), but *w appears in certain cases (possibly through dissimilation when another labial consonant followed?)
Proto-Germanic *hw voiced by Verner's law fell together with this sound and developed identically, compare the words for 'she-wolf': from Middle High German wülbe[citation needed] and Old Norse ylgr, one can reconstruct Proto-Germanic nominative singular *wulbī, genitive singular *wulgijōz, from earlier *wulgwī, *wulgwijōz.
For example, Germanic (word-initial) *b- corresponds regularly to Latin *f-, Greek pʰ-, Sanskrit bʰ-, Slavic, Baltic or Celtic b-, etc., while Germanic *f- corresponds to Latin, Greek, Sanskrit, Slavic and Baltic p- and to zero (no initial consonant) in Celtic.