SOURCE: Germanic */f/
OE /f/ had two different realizations, or allophones, depending upon its phonetic environment.
1. Voiceless labiodental fricative--initially or in the vicinity of voiceless phonemes. Example: fæger "fair."
REFLEX: ME /f/
2. Voiced labiodental fricative--in the vicinity of voiced phonemes. Example: ofer "over."
REFLEX: ME /v/