And instead of getting a pepper-and-salt effect, we find very clear and sharp divisions between the dialects of the United States, which are getting more different from each other as time goes on.

This African American Vernacular English shares most of its grammar and vocabulary with other dialects of English. But it is distinct in many ways, and it is more different from standard English than any other dialect spoken in continental North America.

However, research in the years that followed found that in many of its important features, African American Vernacular English was becoming not less, but more different from other dialects.

William Labov Quotes