Verner's Law (Second Consonant Shift)

 

When surrounded by voiced sounds and preceded by an unaccented vowel,

/f/ (voiceless labiodental fricative) became /b/ (voiced bilabial stop)
/q/ (voiceless interdental fricative) became /d/ (voiced alveolar stop)
/h/ (voiceless velar fricative) became /g/ (voiced velar stop)
/s/ (voiceless alveolar stop) became /r/ (alveolar trill)

 

See also Grimm's Law