Independent Vowel Changes in Proto-Germanic

Change illustrated		Examples
PIE	PG	Non-Germanic	Germanic
			Old	Modern
o o	a а	L nox, Ir nochd, R ночь R могу; мочь	Gt nahts, О Icel nátt, OHG naht Gt magan, OE maʒan, mæʒ	Sw natt, G Nacht Sw må, NE may
aa:	oo:	L mater, R мать О Ind bhrāta, L frater, R брат	О Icel moðir, OE mōdor Gt broðar, O Icel broðir, OE brōðor	Sw moder, NE mother Sw broder, NE brother

In later PG and in separate Germanic languages the vowels displayed a tendency to positional assimilative changes: the pronunciation of a vowel was modi­fied under the influence of the following or preceding consonant; sometimes a vowel was approximated more closely to the following vowel. The resulting sounds were phonetically conditioned allophones which could eventually coincide with another phoneme or develop into a new phoneme.

The earliest instances of progressive assimilation were common Germanic muta­tions; they occurred in Late PG before its disintegration or a short time after. In certain phonetic conditions, namely before the nasal [n] and before [i] or [j] in the next syllable the short [e], [i] and [u] remained or became close (i.e. appeared as [i] and [u]), while in the absence of these conditions the more open allophones were used: [e] and [o], respectively. Later, these phonetic conditions became irre­levant and the allophones were phonologised.

Table 2

Mutation of Vowels in Late pg

Change illustrated	Examples
		Germanic
	Non-Germanic	Old	Modern
P IE G i e e u u o	L ventus, R ветер	Gt winds, O Icel vindr, OE wind	Sw vind, NE wind
	L edit, R ест L edere, R есть	OHG izit, OE itep, O Icel eta, OE etan	G ißt, NE eats, G essen, NE eat
	Lith sunus, R сын	О Icel sunr, OE	Sw son, NE son
		sunu
	Celt hurnan	О Icel, OE horn	NE horn, Sw horn

After the changes, in Late PG, the vowel system contained the following sounds:

SHORT VOWELS i e a o u

LONG VOWELS i: e: a: o: u:¹

¹ IE [a:] became [o:]; the new [a:] developed from short [a] before nasals and also from the open [є:] in West and North Germanic.

It is believed that in addition to these monophthongs PG had a set of diphthongs made up of more open nuclei and closer glides: [ei], [ai], [eu], [au] and also [iu]; nowadays, however, many scholars inter­pret them as sequences of two independent monophthongs.