In speech phonemes are pronounced not by themselves but together with other phonemes in words or at the junction (стык) of words in sentences. In such articulatory continuum they tend to react one upon the other so that the articulation of the phoneme influences the articulation of a neighboring phoneme making it similar to itself. As a result of this process which usually depends on the articulation basis of a language phoneme undergo qualitative changes known as various cases of assimilation.
Assimilation is a general term which mean all kinds of modifications in the pronunciation of a consonants caused by the influence of the neighboring phonemes.
In spite of different typological characteristics of vowels and consonants they almost always slightly influence each other. Such cases of accommodation may be observed in the following example: in the word "tea" the phoneme [t] is pronounced with the spread lips before [ i:], in "tooth" the
phoneme [ t ] is rounded before [ u:], in "tell" the phoneme [ e] is more open before [ 1].
When two identical consonants usually the cacuminal sonant [ r] occur in the neighbouring syllables one of the identical sonants is replaced by the lateral sonant [ 1] e. g. the English word "pilgrim" is derived from "peregrinum", the Russian word "февраль" - from "februarius". Cases of dissimilation may be observed in the illiterate pronunciation, such as "колидор", «секлетарь».
3. Historical and juxtapositional (размещение рядом, сопоставление) assimilation.
The process of historical assimilation took place in the early periods of development of a language. This kind of assimilation usually observed within a word shape, the pronunciation of a word, modified historically, has become its literary standard, e. g. "castle" [ka:sl].
Another example of historical assimilation is the pronunciation of words borrowed from French, such as "occasion", "session", "nature", "question". In these words the consonants [z, s, t] were influenced by  or in the unstressed syllables and the resulting sound were new fricatives [∫,з], and an affricate [t∫].
If the assimilative influence occurs at the junction (соединение) of words it is considered juxtapositional or contextual, it usually appears in careless and slovenly (небрежная) speech. The usage of such assimilations is strictly stylistic and they should be avoided by foreign learners as illiterate pronunciations, e. g. "ten boys" ['tem 'boiz], "could you" [′kυdзju∙].
4. Degree of assimilative changes.
Assimilative changes of articulation may be complete and incomplete. When a change in the articulation of a phoneme results in one of the allophones of the same phoneme the assimilation is incomplete or allophonic, e. g. "month". When on the other hand the assimilative change is so radical (основной, базисный) that the resulting articulation coincides with a different phoneme the assimilation is complete or phonemic. For example, "cupboard"
[′k٨bbәd] - ['k٨bәd].
Very often complete assimilation may lead to the loss of consonants: e. g. "empty" ['em (p) ti], "muscle" ['m٨sl].
5. Physiological aspect of assimilation.
Physiologically assimilative changes involve a change in the position of the tongue "told_ them", the position of the vocal cords "goose" [-s] -"goos_eberry" [-z], the position of the lips "q_uite" and the position of the soft palate "han_dsome".
6. Articulation of plosives.
The articulation of every phoneme pronounced in isolation may be regarded as a complex of three stages: the on set or the on glide (начало), the hold and the off glide.
For the analysis of plosives these stages are termed differently: closure (смыкание), stop and plosion∫∫ thus indicating that the third stage is the most essential one in this type of consonants.
It is not always the case that plosives in English have a third stage consisting of a sudden oral plosion. The main variants are:
1. No audible plosion in clusters of plosives, e. g. "dropped" [-pt-], "big chin" [-gt∫-]. Double closure is formed in the mouth. The closure for the second plosive is made before the plosion of the first. The homogeneous plosives are pronounced together there is only one plosion, the stop stage is doubled, e. g. "top people" [-pp-].
2. Nasal plosion, the compressed air escapes through the nasal cavity, e. g. "topmost" [-pm-], "not now" [-tn-].
3. Lateral plosion, when one or both sides of the tongue are lowered to allow the air to escape, e. g. "middle" [-dl-].
In the speech continuum there may be retained phonetic features which mark word and morpheme boundaries. Thus, the phonetic sequence [pi:sto:ks] may mean "pea stalks" or " peace talks" according to the situation of the word boundaries, according to the type of juncture.
8. Definition of a syllable.
Speech sounds are integral constituent parts of a larger and higher phonetic unit known as the syllable.
As Prof. Trakhterov puts it a syllable is the shortest articulatory unit consisting of one or more phonemes unified by a tonal element and capable of bearing tone and stress. The tonal element is the syllabic impulse produced by a vowel or by a sonorous consonants /m, n, 1, r, ŋ /. It is the inherent (присущая) characteristic of vowels and sonorous consonants and it helps to perceive the whole syllable.
Syllables may consist of a single vowel sound, e. g. "I", "are". Syllables consisting of two or more speech sounds are usually formed by vowels as in "all", "sea" or by sonorous consonants, as in "rhythm".
9. Types of syllables.
From the point of view of syllable division syllables are classified as open, closed, covered and uncovered. The simplest type of a syllable is uncovered and open, e. g. "air". Syllables which end in a consonant are uncovered and closed, e. g. "out", "in". Syllables which begin with a consonant are covered and open, e. g. "now", "tea". Syllables which begin with a consonant and end in a consonant are covered and closed, e. g. "man", "ship".
Structurally syllables in English may be of the following types: CV (consonant + vowel) - /fa:/, CCV - /plei/, CCCV - /skri:m/, VCC - /dоυnt/.
Syllables of one type may not coincide with those of the other. For instance, the word "maker" consists of two syllables both phonetic and orthographic, but orthographic ally the word is divided in the following way -"mak-er", while phonetically the first syllable ends in a diphthong /ei/:/mei-
kә/. The word "name" consists of two open syllables from the viewpoint of spelling and orthography, but it contains only one closed syllable /neim/ from the phonetic point of view.
From the point of view of their duration syllables may be short and long.
From the point of view of their accentual weight syllables may be classified as stressed and unstressed.
10. Morphemic structure of syllables.
The syllabic structure of words has some correlation with their morphemic structure. The syllabic boundary may coincide with the morphemic boundary e. g. "close-ly" /kloυs-li/. But more often the discrepancy (расхождение) between the morpheme and the syllabic boundaries is clearly observed, e. g. "clos-er" /kloυ-sә/.
11. Different approaches to the problem of syllable.
The oldest theory which tries to explain the mechanism of syllable formation is the expiratory theory (теория выдоха) according to which each exhalation corresponds to a new syllable. If that is so then the word "extra" has three syllables, because in this word there are three expiratory pulses in /ek-/, /-s-/, and /-trә/.
A more popularly accepted theory is the sonority theory of Otto Jespersen. According to this theory each sound has its own inherent sonority or carrying power. Sounds of greater sonority (mostly vowels) indicate "peaks" of sonority which are separated from other peaks by "valleys" of lower sonority. Peaks of sonority form syllables, valleys of sonority indicate the place of syllable division. However this theory fails to explain the position of the syllabic boundary in such sequences as, for instance, /aiskri:m/ -whether it is "ice-cream" or "I scream".
The mechanism of syllable formation was instrumentally investigated by Prof. Zhinkin. According to his pharyngeal contraction theory the contraction of the walls of the pharynx (зев) together with the resulting increase in the muscular tension of its walls increase the acctual loudness of vocalic elements which are responsible for the syllable formation.
The theory of the syllable most widely accepted by Russian linguists is the theory of Academician Shcherba. It is based on two principles:
1. the muscular tension or articulatory effort;
2. the type of consonant.
According to this theory each syllable is formed by a new articulatory effort. Syllable division is closely connected with the type of the consonant which may be initially strong e. g. /i:t/, finally strong /ti:/ or geminate (двойной) or double-peaked /٨n_noυn/.