A Thin Line
When Oskar goes into Queens for the first time, he stands on the Fifty-ninth Street Bridge, between Manhattan and Queens, and wonders, "What's the name of the parts of New York […] that aren't in any borough?" (5.6)
It's an invisible boundary, an arbitrary distinction really, between neighborhoods of a city. It's kind of like when you're playing a board game with a spinner and you land "on the line." How do you count it? (It's too confusing for us, so we just spin again.) These invisible boundaries don't just exist between boroughs. They exist between people. And even though he doesn't realize it, Oskar is blurring the lines between people during his quest for the key.
The way he brings people together is most evident when the Blacks attend his school play. Oskar observes:
[The Blacks] must have been half the audience. But what was weird was that they didn't know what they had in common. (7.3)
Oskar doesn't always succeed at bringing people together, not even in his own life. He never gets to know that the renter in Grandma's apartment is his Grandfather, making the scenes when they meet all the more poignant:
[The renter/Grandpa] was on one kind of carpet, I was on another. The line where they came together reminded me of a place that wasn't in any borough. (13.51)
Oskar never does get to cross that line.