In a typological scheme first systematized by Henry M. Hoenigswald in 1965, a historical sound law can only affect a phonological system in one of three ways: This classification does not consider mere changes in pronunciation, that is, phonetic change, even chain shifts, in which neither the number nor the distribution of phonemes is affected.
For example, chain shifts such as the Great Vowel Shift (in which nearly all of the vowels of the English language changed) or the allophonic differentiation of /s/, originally *[s], into [s z ʃ ʒ ʂ ʐ θ χ χʷ h], do not qualify as phonological change as long as all of the phones remain in complementary distribution.
Similarly, in the prehistory of Indo-Iranian, the velars */k/ and */g/ acquired distinctively palatal articulation before front vowels (*/e/, */i/, */ē/ */ī/), so that */ke/ came to be pronounced *[t͡ʃe] and */ge/ *[d͡ʒe], but the phones *[t͡ʃ] and *[d͡ʒ] occurred only in that environment.
Sound changes generally operate for a limited period of time, and once established, new phonemic contrasts rarely remain tied to their ancestral environments.
In most dialects of England, the words father and farther are pronounced the same due to a merger created by non-rhoticity or "R-dropping".
The immediate results are these: For a simple example, without alternation, early Middle English /d/ after stressed syllables followed by /r/ became /ð/: módor, fæder > mother, father /ðr/, weder > weather, and so on.
A trivial (if all-pervasive) example of conditioned merger is the devoicing of voiced stops in German when in word-final position or immediately before a compound boundary (see: Help:IPA/Standard German): There were, of course, also many cases of original voiceless stops in final position: Bett "bed", bunt "colorful", Stock "(walking) stick, cane".
To sum up: there are the same number of structure points as before, /p t k b d g/, but there are more cases of /p t k/ than before and fewer of /b d g/, and there is a gap in the distribution of /b d g/ (they are never found in word-final position or before a compound boundary).
gerō (but perfect gessi < *ges-s- and participle gestus < *ges-to-, etc., with unchanged *s in all other environments, even in the same paradigm).
This sound law is quite complete and regular, and in its immediate wake there were no examples of /s/ between vowels except for a few words with a special condition (miser "wretched", caesariēs "bushy hair", diser(c)tus "eloquent": that is, rhotacism did not take place when an /r/ followed the *s).
(1) a shortening of /ss/ after a diphthong or long vowel: causa "lawsuit" < *kawssā, cāsa "house' < *kāssā, fūsus "poured, melted" < *χewssos.
A particular example of a conditioned merger in Latin is the rule whereby syllable-final stops, when followed by a nasal consonant, assimilated with it in nasality, while preserving their original point of articulation: In some cases, the underlying (pre-assimilation) root can be retrieved from related lexical items in the language: e.g. superior "higher"; Sabīni "Samnites"; sopor "(deep) sleep".
[4] Some epigraphic inscriptions also feature non-standard spellings, e.g. SINNU for signum "sign, insigne", INGNEM for ignem "fire".
That reconstruction makes it easy to unriddle the story behind the weird forms of the Latin paradigm jubeō "order", jussī perfect, jussus participle.
That is, most apparent mergers of A and B have an environment or two in which A did something else, such as drop or merge with C. Typical is the unconditioned merger seen in the Celtic conflation of the PIE plain voiced series of stops with the breathy-voiced series: *bh, *dh, *ǵh, *gh are indistinguishable in Celtic etymology from the reflexes of *b *d *ǵ *g. The collapse of the contrast cannot be stated in whole-series terms because the labiovelars do not co-operate.
A full account of this history is complicated by the subsequent changes in the phonetics of the nasal vowels, but the development can be compendiously illustrated via the present-day French phonemes /a/ and /ã/: Phonemic split was a major factor in the creation of the contrast between voiced and voiceless fricatives in English.
The voiced fricative is typically seen in verbs, too (often with variations in vowel length of diverse sources): gift but give, shelf but shelve.
From a historical perspective, there is no problem since alter is from *alteros (overtly nominative singular and masculine), with the regular loss of the short vowel after *-r- and the truncation of the resulting word-final cluster *-rs.
It seems possible to avoid all those issues by considering loss as a separate basic category of phonological change, and leave zero out of it.
As stated above, one can regard loss as both a kind of conditioned merger (when only some expressions of a phoneme are lost) and a disappearance of a whole structure point.
The ends of words often have sound laws that apply there only, and many such special developments consist of the loss of a segment.
There is in Modern English next to nothing left of the elaborate inflectional and derivational apparatus of PIE or of Proto-Germanic because of the successive ablation of the phonemes making up these suffixes.
However, such stigmatization can lead to hypercorrection, when the dialect speakers attempt to imitate the standard language but overshoot, as with the foot–strut split, where failing to make the split is stigmatized in Northern England, and speakers of non-splitting accents often try to introduce it into their speech, sometimes resulting in hypercorrections such as pronouncing pudding /pʌdɪŋ/.