GA vowels can be divided into two groups: lax vowels, which can only occur in closed syllables (ending in a consonant), and tense vowels, which have no such restriction. This is shown in the table below.
|tense||lax||closed syllables||open syllables|
There are a number of differences between these two classes of vowels.
First, lax vowels tend to be centralized, i.e. pronounced closer to /ə/ than tense vowels.
Second, lax vowels tend to be monophthongal, while tense vowels tend to be diphthongal. That is, even for GA /iː, uː, eɪ, oʊ/, the tongue tends to move off in a closer direction.
Monophthongs are produced with a more or less stationary tongue and lip position, while diphthongs are produced with a movement of the tongue, with or without a change in lip position. Thus, in GA /aɪ/ there is a closing movement of the front of the tongue while the lips are kept unrounded, in GA /aʊ/ there’s a closing movement of the back of the tongue with a simultaneous change from unrounded to rounded lips, while in GA /ɔɪ/ the hump of the tongue ‘rolls’ forward from back to front, while at the same time the lip position changes from rounded to unrounded.
Third, lax vowels tend to be shorter, although for open vowels the length distinction has been neutralized. Thus, while GA /ɪ/ is shorter than GA /iː/ in identical contexts as in bid and bead, /ʌ/ in e.g. mud is almost as long as a /ɑː/ in mod.