The Northern Cities Vowel Shift is a phenomenon in the vowel pattern of much of the United States. This shift has not spread to Canada. It was documented by the American linguist William Labov. It is strongest in cities such as Detroit, Chicago, Buffalo, Rochester and Syracuse. Since these are cities of the northeastern and midwestern United States, the phenomenon has been called the Northern Cities Vowel Shift.
The most notable vowel change in the Northern Cities Vowel Shift concerns the pronunciation of the low back unrounded vowel of words such as "father", "far" and "box". The low back unrounded vowel has shifted so that it is pronounced as either a central low central unrounded or even low front unrounded vowel. Thus the word "fox" sounds similar to the word "fax" to General American speakers.
The low front unrounded vowel raises so that it is pronounced as a lower mid front unrounded lax vowel as in the word "pet". Thus the word "bat" sounds similar to "bet" to General American speakers.
The mid front unrounded lax vowel retracts so that it is pronounced as a central mid unrounded vowel similar to the schwa. The result of this is words such as "pet" and "tell" have a more retracted pronunciation than in General American.
Another change is that the upper mid central unrounded vowel in words such as "but" and "up" is also pronounced with a more retracted pronunciation than in General American. This change is probably the least noticeable of the changes in the Northern Cities Vowel Shift.
In the Northern Cities Vowel Shift, a circular change affects the low vowels. The low back unrounded vowel becomes a more fronted vowel, either a central or front vowel. The low front vowel raises to become a lower mid front unrounded vowel, the lower mid front unrounded vowel becomes a mid central unrounded vowel and the upper mid central unrounded vowel becomes a more retracted vowel. This change has occurred in the pronunciation of many Americans.