This article describes those aspects of the phonological history of the English language which concern consonants.
Consonant clusters
H-cluster reductions
Reduction of /hw/ – to /h/ in a few words (such as who), but usually to , for the great majority of English speakers (so that whine comes to be pronounced the same as wine). Reduction of /hl/, /hr/ and /hn/, with the loss of the initial in Middle English. Reduction of /hj/ to /j/ in a few American and Irish dialects (so that hew is pronounced like yew).
Y-cluster reductions
Yod-dropping – the elision of in certain clusters, depending on dialect (for example, RP has /j/ in new, while General American and Cockney do not). Yod-coalescence, whereby the clusters , , and become , , and respectively (for example, education is often pronounced as if it began "edge").
Other initial cluster reductions
Reduction of /wr/ to /r/, in words like wrap, around the 17th century (there was also a reduction of /wl/ to /l/ in Middle English). Reduction of /kn/ and /ɡn/ to /n/, in words like knot and gnome, around the 17th century. S-cluster reduction, in some types of Caribbean English, where for example spit is pronounced pit.
Final cluster reductions
NG-coalescence – reduction of the final cluster to , in words like hang, which has occurred in all but a few English dialects. G-dropping – reduction of the final cluster to in weak syllables, principally in the verb ending -ing, which has occurred in many English dialects, although not in the modern standard varieties. Reduction of /mb/ and /mn/ to , in later Middle English, affecting words like lamb and column. Generalized final cluster reduction in African American Vernacular English (AAVE) and Caribbean English, where for example desk and hand may be pronounced "dess" and "han".
Other changes involving clusters
Reduction of /ts/ to /s/ – a Middle English reduction that produced the modern sound of soft . Medial cluster reduction – elision of certain stops in medial clusters, such as the /t/ in postman. Insertion (epenthesis) of stops after nasals in certain clusters, for example making prince sound like prints, and dreamt rhyme with attempt. Assimilation of certain consonants in clusters, especially nasals. Glottalization and pre-glottalization (insertion of a glottal stop in place of or before a /t/ or other stop) in certain environments, depending on dialect. Certain other changes occurring in AAVE, including S-cluster metathesis (as with the use of "aks" for ask – an alternation which also has a long history in general forms of English), the merger of /str/ and /skr/, and yod-rhotacization (where beautiful is pronounced "brootiful").
Stops
Aspiration
The voiceless stops /p/, /t/, /k/ are typically aspirated when they begin a stressed syllable, becoming , as described under English phonology (obstruents). There is some regional variation in the degree of aspiration, and in some Scottish and northern English accents aspiration does not occur at all.
In certain accents, such as Geordie (among younger women) and in some speakers of Dublin English, /p/, /t/ and /k/ can be preaspirated when they come at the end of a word or utterance, becoming .
Flapping
Flapping, or tapping, is a process whereby /t/ or /d/ is pronounced as the alveolar flap in certain positions, especially between vowels (but also sometimes after other sonorants). It may be perceived as, for example, the pronunciation of butter as "budder". It occurs especially in North American English (to varying extents) and in Australian and New Zealand English.
Voicing
Apart from the T-voicing that results from flapping (described above), some dialects feature other instances of voicing or lenition of the stops /p/, /t/ and /k/. In Geordie, these stops may be fully voiced ([b], [d], [ɡ]) in intervocalic position. In Devon, stops and other obstruents may be voiced (or at least lenited) between vowels and when final after a weak vowel, so for example the /k/ and /t/ in jacket may approach the realizations [ɡ] and [d], making the word sound similar or identical to jagged.
Glottalization
Stops, chiefly the voiceless stops, and especially /t/, are frequently glottalized or pre-glottalized in certain positions; that is, a stop may be replaced with the glottal stop , or else a glottal stop may be inserted before it. These phenomena are strongly dependent on the phonetic environment and on dialect. For details, see T-glottalization, as well as English phonology (obstruents) and glottalization in consonant clusters.