- Alternative form of all of you. Plural form of you, including everyone being addressed.
- I'm glad you all came here today.
- (idiomatic, chiefly Southern US, colloquial) Plural form of you or singular formal form of you.
- You all are driving that truck?
In US South usage, you all or y'all is used to address either more than one person, or one person whom the speaker does not know personally or well, or who is much older or as a sign of respect for the person's position or seniority. You all or y'all may also be used even for a single person the speaker knows well, but in a formal environment such as a classroom or when asking a question at a public meeting. This usage is very much like that of the French vous, and similar to the German Sie.
Many US Southerners will deny that you all or y'all is ever properly used in the singular, but rather it appears to be used that way occasionally when there is an "implied" plural you (a "corporate" you, a "team" you, or "you and your family", "you and your friends" (who aren't here right now), "you and your party", etc.).