Boston (Mostly Southie)

Like the classic film Good Will Hunting, this movie takes place in the working-class Boston neighborhood known as Southie. Almost every character tends to mention this fact at one point or another, because everybody knows that when someone says Southie, they really mean "rough and tough." And rough and tough is what you want to be in this world.

For example, Sergeant Dignam connects a dead mob guy to Frank Costello's gang simply by saying: 

DIGNAM: You know where that dirtball started his life? Southie projects.

His partner Captain Queenan is also quick to notice that Billy Costigan is from this area, saying: 

QUEENAN: So you have family connections down in Southie, right? Through your father?

This is exactly why Billy will make a great undercover cop: being from Southie isn't something a person can easily fake.

One of the things that this movie establishes about Southie is that it's a community with really deep roots. The Irish mob only trusts Billy Costigan because his family goes back many generations in the neighborhood, and the men in his family (except his dad) have all been involved in the Irish mob.

There's a strong sense of loyalty among the men in this neighborhood, but there's also a lot of fear and respect, as we see when Billy's cousin Sean finishes a drug deal by saying: 

SEAN: We're not even supposed to be doing this s*** this close to Worcester. This side of Worcester.

That means that there's a very specific street in the Southie neighborhood that people aren't supposed to cross without getting Frank Costello's permission.

So yeah, it's a small world when you live in Southie. It's a world of deep loyalty and community, but it's also a lot of violence for anyone who steps out of line or doesn't play by the community's rules.