south. However, it is possible that this change already occurred in Proto-Germanic proper, in which case the phenomenon would have remained merely allophonic for quite some time. If that is the case, that would be the stage reflected in Gothic, where there is no orthographic evidence of i-mutation at all. Long vowels and diphthongs were affected only later, probably analogically, and not in all areas. Notably, they were not mutated in most (western) Dutch dialects, whereas short vowels were. a-mutation: is lowered to when a non-high vowel follows in the next syllable. This is blocked when followed by a nasal followed by a consonant, or by a cluster with in it. Hence PG > OE/ModE gold, but PG > OE gyldan > ModE gild. This produces a new phoneme , due to inconsistent application and later loss of word-final vowels. Final-syllable long vowels were shortened. Final becomes , later raised to . PG ("saw (tool)") > OE sagu, ON sǫg. Final becomes in ON (later raised to ), in West Germanic. PG ("he/she/it healed") > ON heilði, but OE hǣlde, OHG heilta. The final long diphthong loses its final element and usually develops the same as from that point on. PG ("gift", dative singular) > NWG > ON gjǫf, OHG gebu, OE giefe (an apparent irregular development). "Overlong" vowels were shortened to regular long vowels. PG (maybe already by late PG) becomes . This preceded final shortening in West Germanic, but postdated it in North Germanic. Unstressed diphthongs were monophthongized. > , > . The latter merged with ō from shortened overlong ô. PG ("son", genitive singular) > NWG > ON sonar, OE suna, OHG suno; PG ("he/she/it take", subjunctive) > NWG > ON nemi, OE nime, OHG neme; PG ("stone", dative singular) > NWG > ON steini, OE , OHG steine. West Germanic period This period occurred around the 2nd to 4th centuries. It is unclear if there was ever a distinct "Proto-West Germanic", as most changes in this period were areal, and likely spread throughout a dialect continuum that was already diversifying further. Thus, this "period" may not have been a real timespan, but may simply cover certain areal changes that did not reach into North Germanic. This period ends with the further diversification of West Germanic into several groups before and during the Migration Period: Ingvaeonic, Istvaeonic (Old Frankish) and Irminonic (Upper German). Loss of word-final .