Contents Introducti

partial assimilation is distinguished. When one or more features of the phoneme are affected, we have partial assimilation as in tree, for example, [t] becomes post-alveolar under the influence of the following [r], but it retains its distinctive features and is still occlusive, forelingual and fortis. When all the features of the phoneme are adapted, we have a case of complete assimilation, for example, in rapid casual speech [t] will become [p] before a bilabial consonant, as in that person: [ðæpˈpɜ:sn].
Such cases of assimilation create something of a problem for the phoneme theory, as it’s not clear whether one phoneme has been substituted for another phoneme or one of its allophones. Traditionally, if a phoneme loses one of its distinctive features it is said to be a different phoneme, phonemes are supposed not to overlap in their allophones. Nowadays some phoneticians don’t consider this restriction important any more and regard these sounds as allophones of one phoneme.
1.2. Vowel-consonant distinctions

I just want to tell that it is important to know distinctions between vowels

and consonants. We try to show some distinctions between them in this chapter. We discussed about vowels enough above and now we should define what a consonant is. A consonant is a speech sound made by partially or completely blocking the flow of air through the mouth (using the lips, teeth, tongue, and palate). Letters of the English alphabet that represent consonants include all the letters that are not vowels The letter “y” makes a consonant sound when it appears at the beginning of words (examples: yellow, yacht), and it makes a vowel sound when it appears at the end of words (examples: valley, fairy).The distinction between them is discussed below. Consonants and vowels are two different qualities of sounds that are found almost in all languages of the world. The distinctions between them are discussed below.
Consonants are defined as the sounds articulated by temporary obstruction in the air stream which passes through the mouth. The obstruction made by the articulators may be 'total', 'intermittent', 'partial', or may merely constitute a narrowing sufficient to cause friction. In the articulation of consonants almost all articualtors are involved. Especially the position of the soft palate causes the division of consonants into 'oral consonants' and 'nasal consonants'. when the soft palate is raised, 'oral consonants' are produced; the soft palate is lowered, 'nasal consonants' are produced. In English /m/, /n/, and / /are nasal consonants and rest of all are oral consonants. The function of vocal cords also causes the division of consonants as 'voiceless' and 'voiced'. When vocal cords are kept apart, voiceless consonants as /p, t, k, f, s, / are produced whereas their vibrations produce voiced consonants as /b, d, g, v, z/. But vowels are the sounds that are produced with an approximation without any obstruction in the air passage. Among all articulators, only tongue is prominent in their production.

All vowel sounds are voiced and all of them are 'oral' as during the

production of them the soft palate is raised and hence the nasal cavity is completely blocked. The examples of the vowels are /i, e, a, u, y, o, /.
Consonants in phonetics are referred to as 'contoids' which often appear as the marginal elements in the 'syllable'. They seldom form nucleus of the syllable except some case. The consonants 'n' and 'l' in the second syllable of the words 'button' and 'apple' form nucleus. But vowels referred to as 'vocoids' in phonetics always form the nucleus of the syllable as in 'bill', 'pill', 'mill', 'heat', etc. Even in their manner of classification, consonants and vowels show apparent distinction. Consonants are identified or classified in terms of 'voicing', 'place of articulation', and 'manner of articulation', whereas vowels in terms of the 'height of tongue', 'part of tongue', which is raised or lowered, and 'lip rounding'.

Classification of consonants

Consonants as discussed above are classified in terms of: 1. voicing
2. place of articulation
3. manner of articulation Voicing.
On the basis of voicing, consonants are divided into 'voiced consonants' and 'voiceless consonants'. Voiced consonants are those which are articulated with the vibration of the vocal cords. In English voiced consonants are /b, d, g, j, v, , z/. Voiceless consonants are articulated without vibration of vocal cords or it may be said that during the production of voiceless consonants vocal cords are kept apart. Examples: /p, t, k, f, s, h/.