Phonological change

In historical linguistics, phonological change is any sound change that alters the distribution of phonemes in a language. In other words, a language develops a new system of oppositions among its phonemes. Old contrasts may disappear, new ones may emerge, or they may simply be rearranged. Sound change may be an impetus for changes in the phonological structures of a language. One process of phonological change is rephonemicization, in which the distribution of phonemes changes by either addition of new phonemes or a reorganization of existing phonemes. Mergers and splits are types of rephonemicization and are discussed further below.

Types

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:

Conditioned merger, in which some instances of phoneme A become an existing phoneme B; the number of phonemes does not change, only their distribution.
Phonemic split, in which some instances of A become a new phoneme B; this is phonemic differentiation in which the number of phonemes increases.
Unconditioned merger, in which all instances of phonemes A and B become A; this is phonemic reduction, in which the number of phonemes decreases.

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.

Phonetic vs. phonological change

can occur without any modification to the phoneme inventory or phonemic correspondences. This change is purely allophonic or subphonemic. This can entail one of two changes: either the phoneme turns into a new allophone—meaning the phonetic form changes—or the distribution of allophones of the phoneme changes.
For the most part, phonetic changes are examples of allophonic differentiation or assimilation; i.e., sounds in specific environments acquire new phonetic features or perhaps lose phonetic features they originally had. For example, the devoicing of the vowels and in certain environments in Japanese, the nasalization of vowels before nasals, changes in point of articulation of stops and nasals under the influence of adjacent vowels.
Phonetic change in this context refers to the lack of phonological restructuring, not a small degree of sound change. For example, chain shifts such as the Great Vowel Shift or the allophonic differentiation of /s/, originally, into, do not qualify as phonological change as long as all of the phones remain in complementary distribution.
Many phonetic changes provide the raw ingredients for later phonemic innovations. In Proto-Italic, for example, intervocalic */s/ became *. It was a phonetic change, merely a mild and superficial complication in the phonological system, but when * merged with */r/, the effect on the phonological system was greater.
Similarly, in the prehistory of Indo-Iranian, the velars */k/ and */g/ acquired distinctively palatal articulation before front vowels, so that */ke/ came to be pronounced * and */ge/ *, but the phones * and * occurred only in that environment. However, when */e/, */o/, */a/ later fell together as Proto-Indo-Iranian */a/, the result was that the allophonic palatal and velar stops now contrasted in identical environments: */ka/ and /ča/, /ga/ and /ǰa/, and so on. The difference became phonemic.
Sound changes generally operate for a limited period of time, and once established, new phonemic contrasts rarely remain tied to their ancestral environments. For example, Sanskrit acquired "new" /ki/ and /gi/ sequences via analogy and borrowing, and likewise /ču/, /ǰu/, /čm/, and similar novelties; and the reduction of the diphthong */ay/ to Sanskrit /ē/ had no effect at all on preceding velar stops.

Merger

Phonemic merger is a loss of distinction between phonemes. Occasionally, the term reduction refers to phonemic merger. It is not to be confused with the meaning of the word "reduction" in phonetics, such as vowel reduction, but phonetic changes may contribute to phonemic mergers. For example, in most North American English dialects, the vowel in the word lot and vowel in the word palm have become the same sound and thus undergone a merger. 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".

Conditioned merger

Conditioned merger, or primary split, takes place when some, but not all, allophones of a phoneme, say A, merge with some other phoneme, B. The immediate results are these:

there are the same number of contrasts as before.
there are fewer words with A than before.
there are more words with B than before.
there is at least one environment for which A, for the time being, no longer occurs, called a gap in the distribution of the phoneme.
if inflection or derivation result in A sometimes but not always being in the environment in which it has merged with B, an alternation in that environment between A and B may develop.
Example from Middle English

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. Since /ð/ was already a structure-point in the language, the innovation resulted merely in more /ð/ and less /d/ and a gap in the distribution of /d/.

Devoicing of voiced stops in German

A trivial example of conditioned merger is the devoicing of voiced stops in German when in word-final position or immediately before a compound boundary :

*hand "hand" > /hant/
Handgelenk "wrist" /ˈhantɡəlɛŋk/
*bund "league, association" > /bʊnt/
*gold "gold" > /gɔlt/
*halb "half" > //
halbamtlich "semi-official" /ˈhalpʔamtlɪç/
*berg "mountain" /bɛɐ̯k/
*klug "clever, wise" > /kluːk/

There were, of course, also many cases of original voiceless stops in final position: Bett "bed", bunt "colorful", Stock " 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/.

Rhotacism in Latin

More typical of the aftermath of a conditioned merger is the famous case of rhotacism in Latin : Proto-Italic *s > Latin /r/ between vowels: *gesō "I do, act" > Lat. gerō.
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. However, a new crop of /s/ between vowels soon arose from three sources. a shortening of /ss/ after a diphthong or long vowel: causa "lawsuit" < *kawssā, cāsa "house' < *kāssā, fūsus "poured, melted" < *χewssos. univerbation: nisi "unless" < the phrase *ne sei, quasi "as if" < the phrase *kʷam sei. borrowings, such as rosa "rose" /rosa/, from a Sabellian source, and many words taken from or through Greek.

Nasal assimilation and "gn" in Latin

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:

*supimos > *supmos > summus "highest"
*sabnyom > Samnium "Samnium"
*swepnos > somnus "sleep"
*atnos > annus "year"

In some cases, the underlying root can be retrieved from related lexical items in the language: e.g. superior "higher"; Sabīni "Samnites"; sopor " sleep". For some words, only comparative evidence can help retrieve the original consonant: for example, the etymology of annus "year" is revealed by comparison with Gothic aþna "year".
According to this rule of nasal assimilation, the sequences *-g-n and *-k-n would become, with a velar nasal :

*dek-no- > dignus "worthy"
*leg-no- > lignum "firewood"
*teg-no- > tignum "timber"
*agʷnos > *ag-nos > agnus "lamb"

The sound was not a phoneme of Latin, but an allophone of /g/ before /n/.
The sequence was regularly rendered in the orthography as |gn|. Some epigraphic inscriptions also feature non-standard spellings, e.g. SINNU for signum "sign, insigne", INGNEM for ignem "fire". These are witness to the speakers' hesitancy on how to best transcribe the sound in the sequence.
The regular nasal assimilation of Latin can be seen as a form of "merger", insofar as it resulted in the contrast between oral stops and nasal stops being regularly neutralized.

Concerning the number of contrasts

One of the traits of conditioned merger, as outlined above, is that the total number of contrasts remains the same, but it is possible for such splits to reduce the number of contrasts. It happens if all of the conditioned merger products merge with one or another phoneme.
For example, in Latin, the Pre-Latin phoneme *θ disappears as such by merging with three other sounds: *f, *d, and *b:
Initially *θ > f:

PItal. *tʰi-n-kʰ- "model, shape" > *θi-n-χ- > Lat. fingō
PItal. *tʰwor- "door" > *θwor- > Lat. forēs "door"

Medially adjacent to *l, *r, or *u, *θ becomes b:

PItal. *wertʰom "word" > *werθom > *werðom > Lat. verbum
PItal. *rutʰros "red" > *ruθros > *ruðros > Latin ruber, cf. rubra fem. rubrum neut.
PItal. *-tʰlo-/*-tʰlā- "tool suffix" > Latin -bulum, -bula: PIE *peH₂-dhlo- "nourishment" > PItal. *pā-tʰlo- > *pāθlo- > Latin pābulum; PIE *suH-dhleH₂- "sewing implement" > PItal. *sūtʰlā > *sūθlā > Latin sūbula "cobbler's awl"

Elsewhere, *θ becomes d:

PItal. *metʰyo- "middle" > *meθyo- > Pre-Lat. *meðyo- > Lat. medius
PItal. *pʰeytʰ- > *feyθ- > *feyð- > Lat. fīdus "trusting"

There is no alternation to give away the historical story, there, via internal reconstruction; the evidence for these changes is almost entirely from comparative reconstruction. That reconstruction makes it easy to unriddle the story behind the weird forms of the Latin paradigm jubeō "order", jussī perfect, jussus participle. If the root is inherited, it would have to have been PIE *yewdh-.