Theory of phonetics

[x] being classed as historically short tends to be lengthened in Modern English, especially before lenis consonants [b], [d], [g], [G], [m], [n], [z]. In this position [x] has the same quantity as long vowels [i:], [Q:], [O:], [u:], [E:]. This extra length serves an additional distinctive feature and the qualitative – quantitative relation of [x] – [e] tends to become of the same type as [i:] – [I].
Tenseness characterizes the state of our organs of speech at the moment of production of a vowel. Historically long vowels are tense while historically short vowels are lax. Tenseness may be considered as indispensable concomitant feature of English long vowels.
Thus there are two characteristics that are functionally relevant. They are:

1. 
   stability of articulation;
   
2. 
   tongue position.
   

The rest of the features mentioned above, that is lip position, character of vowel end, length, and tenseness, are indispensable constituents of vowel quality.
The unstressed syllables are usually associated with vowels of central or centralized quality [q], [I], sometimes [V] and the diphthongs [EV], [aI] (or a syllabic consonant). For example, about [q'baut], restore [rI'stO:], hopeful ['hqupfVl], connect [kq'nekt], aubergine ['qVbqGi:n], garden ['gQ:dn].
Also vowels of full quality sometimes occur in unstressed positions, often in borrowed words of Latin and Greek origin, e.g. architect ['Q:kItekt], paragraph ['pxrqgrQ:f], canteen [kxn'ti:n]. In English as well as in Russian there are numerous alternations of vowels in stressed and unstressed syllables between the derivatives of the same root or different grammatical forms of the same word:
[x] – [q] man – sportsman
[A] – [q] some – something
[P] – [q] combine n - combine v
[eI] –[q] operation - operative
[EV]–[q] post – postpone

We may represent English vowels in the following way:

height		row
		front	front retracted	central (or mixed)	back advanced	far back
high	narrow	i: