Verner's Law

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A law stating essentially that Proto-Germanic noninitial voiceless fricatives in voiced environments became voiced when the previous syllable was unstressed in Proto-Indo-European. For example, both the th- and the -d of English third are descended from Proto-Germanic voiceless *th, but the second was voiced by Verner's Law.