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y'all" is modern English's answer to a second-person plural pronoun.
"All y'all" (or "all of y'all") on the other hand, is an occasional variation some use to mean a group of people (as opposed to just two or
three).
For example:
When speaking to two or
three people: "Are y'all going to the movies?"
When speaking to several people: "Are all y'all going to the movies?"
Things are further complicated when using the possessive form of the word. For example:
"Is this y'all's car?"
"Is this all y'all's favorite color?"
Note, though, that there is some debate on the spelling of the possessive form of "y'all." Some will spell it "y'all's" whereas others will spell it "y'alls.""
Cheers
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