There were four voiceless frikative consonants in Proto-Germanic – /f/, /þ/, /h/ and /s/. These voiceless frikatives changed into their voiced counterparts – /v/, /ð/, /g/ and /z/ – with one exception (which was discovered by the linguist Karl Werner): They remained voiceless when they were immediately preceded by the accent. This happened before the accent moved to the first syllable in all words.
In ON. the alternation has been evened out in many of the affected verbs. The alternations /f/ : /v/ is nowhere to be seen, but the others can still be seen in a few words.
The alternation /þ/ : /ð/ is seen only where /þ/ was assimilated with the preceding consonant, and /ð/ was not. The PGerm. *finþana 'to find' - *fanþ - *funðun - *funðenaz has become the ON finna - fann - fundu - fundinn.
The alternation between /h/ and /g/ has in ON become an alternation between nothing and /g/, since most /h/:s have disappeared. The PGerm. *slahana 'to beat' - *slôh - *slôgun - *slagenaz has in ON become slá - sló - slógu - slaginn.
Others: flá 'to skin', sjá 'to see', þvá 'to wash'.
The /s/ : /z/ alternation has turned into an alternation /s/ : /r/, and *wesana 'to be' - *was - *wêzun - *wazenaz has in ON become vesa - vas - váru - verit (neut.).
Others: kjósa 'to choose'.
© Peter Pettersson