Subtheme 1. Phonology

The oldest Germanic languages all share a number of features, assumed to be inherited from Proto-Germanic. Phonologically, this includes the important sound changes known as Grimm's Law and Verner's Law, which introduced a large number of fricatives; late Proto-Indo-European (PIE) had only one, /s/.

The main vowel developments are the merging (in most circumstances) of long and short /a/ and /o/, producing short /a/ and long /ō/. This likewise affected the diphthongs, with PIE /ai/ and /oi/ merging into /ai/ and PIE /au/ and /ou/ merging into /au/. PIE /ei/ developed into long /ī/. PIE long /ē/ developed into a vowel denoted as /ē₁/ (often assumed to be phonetically [ǣ]), while a new, fairly uncommon long vowel /ē₂/ developed in varied and not completely understood circumstances. Proto-Germanic had no front rounded vowels, although all Germanic languages except forGothic subsequently developed them through the process of i-umlaut.

Proto-Germanic developed a strong stress accent on the first syllable of the root (although remnants of the original free PIE accent are visible due to Verner's Law, which was sensitive to this accent). This caused a steady erosion of vowels in unstressed syllables. In Proto-Germanic this had progressed only to the point that absolutely final short vowels (other than /i/ and /u/) were lost and absolutely final long vowels were shortened, but all of the early literary languages show a more advanced state of vowel loss. This ultimately resulted in some languages (e.g., modern English) losing practically all vowels following the main stress and the consequent rise of a very large number of monosyllabic words.