If a Hitchhiker were to ever find themselves travelling in the south of the United States, they would probably notice two things right off; the first being the ridiculous amount of Waffle House restaurants they would see; the second being the local dialect that is spoken throughout Georgia, Alabama, the Carolinas, Texas, etc1.
One of the mainstays of this vocabulary is the use and understanding of the second person plural word 'y'all' and its variants. 'Y'all' is a contraction of sorts, a shortening of the phrase 'you all'. As one may note, even the long form of this phrase stands on shaky grammatical footing, but lets face it, Americans have a tendency to bend the rules where grammar is concerned; sometimes for the better, sometimes for the worse. And sometimes just because they feel like it.
'Y'all' should be used to address a smallish group of people; between one and five. It is an all-inclusive phrase in its ambiguity. It certainly comes in handy when you are addressing a group and you don't want to use Yankee equivalents such as the New York 'youse' or the Boston 'you guys'.
Now, if you are addressing a larger group of people, the phrase 'all y'all' should be used. 'All y'all' is, of course 'all of you all' which makes absolutely no sense, but as the subject of the great American respect for proper grammar and spelling has been already been raised, it shall be left at that. An example of this would be if addressing a larger group of people. If one were, say, addressing a crowd of rowdy students, the following would be completely in order: 'All y'all better listen up!'.
Certainly this Researcher had more than a little problem with it when he moved below the Mason-Dixon line. But over the years, the phrase grew on him to such an extent, that he now harbours a deep affection for it, and even likes using it when he can.