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