The introduction to this article provides insufficient context for those unfamiliar with the subject matter. Please help improve the introduction to meet Wikipedia's layout standards. You can discuss the issue on the talk page.

H-cluster reductions

The wine-whine merger is the merger of /ʍ/ or /hw/ (spelt wh) with /w/. It occurs in the speech of the great majority of English speakers.
The hole-whole merger is the replacement of /ʍ/ with /h/ before the vowels /oː/ and /uː/ which occurred in Old English. Wh-cluster reductions
The yew-hew merger is a process that occurs in some dialects of English that causes the cluster /hj/ to be reduced to /j/.

Yew-hew merger
The hl-cluster, hr-cluster and hn-cluster reductions are three reductions that occurred in Middle English that caused the consonant clusters /hl/, /hr/ and /hn/ to be reduced to /l/, /r/, and /n/. For example, Old English hlāf, hring and hnutu became loaf, ring and nut in Modern English.

hl-cluster, hr-cluster and hn-cluster reductions

Y-cluster reductions
Yod-dropping is the elision of the sound [j]. The term comes from the Hebrew letter yod, which represents [j].
Yod-dropping before [uː] occurs in most varieties of English in the following environments:
In yod-pronouncing dialects, the spellings eu, ew, ute, ue and ui, as in feud, few, mute, cue and suit generally indicate /juː/ or /ɪu/, while the spellings oo and ou, as in moon and soup generally indicate /uː/.

After [tʃ, dʒ, j], for example chew [tʃuː], juice [dʒuːs], yew [juː]
After /ɹ/, for example rude [ɹuːd]
After consonant+/l/ clusters, for example blue [bluː]
After /s/, for example suit [suːt]
After /l/, for example lute [luːt]
After /z/, for example Zeus [zuːs]
After /θ/, for example enthusiasm [ɛnˈθuːziæzəm]
After /t/, /d/ and /n/, for example tune [tuːn], dew [duː], new [nuː] Yod-dropping
Yod-coalescence is a process that changes the clusters [dj], [tj], [sj] and [zj] into [dʒ], [tʃ], [ʃ] and [ʒ] respectively.
This generally occurs in unstressed syllables in all varieties of English, except for the older RP varieties. Occurring in unstressed syllables, it leads to pronunciations such as the following:
It also occurs in some accents in stressed syllables as in tune and dune. Yod-coalescence in stressed syllables occurs in Australian, Cockney and Estuary English. Yod-coalescence has traditionally been considered nonRP, and thus not used by RP speakers.
See also

List of yod-dropping and coalescence homophones Yod-coalescence

Rap-wrap merger
The not-knot merger is a reduction that occurs in modern English where the historical cluster /kn/ is reduced to /n/ making knot and not homophones. This reduction is complete in present English, although it has not happened in all varieties of Scots.

Not-knot merger
The nome-gnome merger is the reduction of the initial cluster /gn/ to /n/ that occurs in all dialects of present English. In Middle English, words spelt with gn like gnat, gnostic, gnome etc. had the cluster /gn/.

Nome-gnome merger
S-cluster reduction is the dropping of /s/ from the initial consonant clusters with voiceless plosives (environments /sp/, /st/, and /sk(ʷ)/) occurring in Caribbean English. After the initial /s/ is removed, the plosive is aspirated in the new word-initial environment, resulting in pronunciations such as:

S-cluster reduction

Final cluster reductions
Final consonant cluster reduction is the nonstandard reduction of final consonant clusters in English occurring in African American Vernacular English and Caribbean English. The new final consonant may be slightly lengthened as an effect.
Examples are:
The plural of test and desk become tesses and desses by the same English rule that gives us plural messes from singular mess.

Nonstandard final consonant cluster reduction
The plum-plumb merger is the reduction of the final cluster /mb/ to /m/ that occurs in all dialects of present English. In early Middle English, words spelt with mb like plumb, lamb etc. had the cluster /mb/.

Plum-plumb merger

Consonant cluster alterations
Yod-rhotacization is a process that occurs for some Southern AAVE speakers where /j/ is rhotacized to /r/ in consonant clusters causing pronunciations like:

Yod-rhotacization
S-cluster metathesis is the nonstandard metathesis of final consonant clusters starting with /s/ occurring in African American Vernacular English. and can still be found in some dialects of English including, of course, African American Vernacular English. It is, however, one of the most stigmatized features of AAVE, often commented on by teachers. It also persists in Ulster Scots as /aks/ and Jamaican English as /aːks/, from where it has entered the London dialect of British English as /ɑːks/.

Scream-stream merger

Phonological history of the English language
Phonological history of English consonants

• G-dropping
  Phonological history of English fricatives
  
  
  
  • H-dropping