The following diagram illustrates the shifts in vowels that have occured in varieties of Southern States English and African-American English:
As the diagram shows, there are a number of shifts among the vowels:
in words such as meet is shifting back and downwards
in words such as mitt is shifting upwards and forwards, often becoming diphthongized, as is common among tense vowels in Standard English
in words such as mate is shifting back and downwards
in words such as met is shifting upwards and forwards, becoming diphthongized, as is common among tense vowels in Standard English
and 
in words such as moot and mote are shifting forwards
According to Wolfram and Schilling-Estes (1998), this shift is largely confined to rural areas of the areas of the Southern States traditionally defined as the South.
Feagin (1986) did a study of the Southern Vowel shift among speakers of English in Anniston, Alabama. The samples were recorded and the F1 and F2 formants plotted on graphs such as those below (on the graph below, move the mouse over any vowel to see that area highlighted):
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As you can see from the graph above of the measurements taken of an older working-class male, the back vowels /u/ and /uw/ are very far forward, particularly in comparison with other back vowels such as /o/ and ow/. This illustrates the back shift in the Southern Vowel Shift. The front vowels, however, do not seem to be undergoing the vowel shift to a large extent. The /e/ vowel has definitely raised above the /ey/ vowel, indicating the front vowel shift, but the /iy/ and /i/ vowels have not shifted greatly.
Now, for comparison, observe the following graph of a teenage male from the same area:
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As you can see from this graph, the back vowels have again shifted forward, and in some tokens of /uw/ are in the extreme front. The front shift is even more pronounced in this speaker, with a few tokens of /iy/ (namely she and keeps) still in the upper front region, but most tokens below and behind the /i/ region. The vowel /ey/ is also back and below the vowel /e/.
Feagin theorizes, based on findings like those above and others, that the back vowel shift is an older shift, since it is found in both older and younger speakers. The front shift, on the other hand, is a more recent shift, occuring more dramatically in the younger speakers.
© 2001 The Language Samples Project