In historical linguistics, lexical diffusion is both a phenomenon and a theory. The phenomenon is that by which a phoneme is modified in a subset of the lexicon, and spreads gradually to other lexical items. For example, in English, has changed to in good and hood but not in food; some dialects have it in hoof and/or roof but others do not; in flood and blood it happened early enough that the words were affected by the change of to , which is now no longer productive.