Professor Labov argues that black Americans have become more monolingual since the 60's - that fewer of them have a mastery of standard English. That's the result of residential segregation, the fact that poor blacks tend to live with poor blacks. But it's also compounded by desegregation, which ended up separating the black poor and the black middle class.

Because of these two factors, there's now a large group of poor black people whose face-to-face conversations are almost entirely with people like themselves. As the cultural critic Greg Tate told me, black people are "segregated, landlocked and institutionalized between prison, the project and public institutions." He added that "there's a certain tribal caste to segregated African-American communities for that reason," and that's reflected in their increased monolingualism.

Writing in The Times 25 years ago, James Baldwin ventured that the black vernacular was one of self-defense. "There was a moment, in time, in this place," he recalled, "when my brother, or my mother, or my father, or my sister, had to convey to me, for example, the danger in which I was standing from the white man standing just behind me, and to convey this with a speed and in a language, that the white man could not possibly understand, and that, indeed, he cannot understand, until today."

Is that still true? The black vernacular seems to be everywhere these days, from Dave Chappelle's show to Boost Mobile's "Where you at?" ad campaign.

"It becomes part of the mainstream in a minute," the poet Amiri Baraka told me, referring to the black vernacular. "We hear the rappers say, 'I'm outta here' - the next thing you know, Clinton's saying. 'I'm outta here.' " And both Senator John Kerry and President Bush are calling out, "Bring it on," like dueling mike-masters at a hip-hop slam.