Who says ‘y’all’? Where are they from and what do people who dont say it think of it?
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From what I understand, most dialects work as you describe: you = singular and y'all = plural.
There is some controversy over whether some dialects have extended this further, such that y'all = singular and "all y'all" = plural.
Here is a discussion over at Language Log, where they say that there is a lot of disagreement about this. I think the overall sense from this article is that people have anecdotes and random quotes where people use y'all as a singular, but no person from the South who attests that "yes, this is what I do."
And here is a followup discussion. Here there is a similar type of disconnect between anecdote and speaker intuition. There seems to be mention that in Oklahoma, y'all can be used for singular and plural, which, if true, might be fueling a false conclusion about "all y'all" being the plural of singular y'all.
There is a lot more in there, and it is worth a read for anyone interested, but the last thing I wanted to mention was this hypothesis at the end of that page:
Thomas Nunnally (1994) has offered a second hypothesis for the emergence of yall as a singular. He suggests that it may well be expanding to fill the role of a polite singular, just as you did several centuries ago. He points out that many of the citations of yall-singular show the form occurring at the edges of discourse-in greetings, partings, and so forth. The following citation, provided to us by Robin Sabino (1994), certainly fulfills this function. Sabino overheard an African-American waitress in an Opelika, Alabama, restaurant say to a customer eating alone, "How are you-all's grits?"
All of this may seem strange, but if you look at the origin of you itself, the same thing happened: it used to be that thou/thee was 2nd person singular and ye/you was 2nd person plural, but as we know, plural you became the 2nd person pronoun for singular and plural (in Standard English, at least). So these kinds of shifts are possible.
Note: This has been extensively edited in light of some research I found over at Language Log.
This is the way I always explain it. You is singular. "Are you going to lunch after church?"meaning you yourself, singular. Y'all is plural. "Are y'all going to lunch after church? meaning is any of your group going. All y'all is what I call plural inclusive. "Are all y'all going to lunch after church?"meaning is every single last one of your group going. Important details when you need a head count to reserve a table.