The Berlin Wall is at the center of this book, both literally and figuratively. When the narrative begins, it's been up for about a year, separating Berlin into two cities: East Berlin and West Berlin. It's forbidden to cross it, and although a few characters in the book attempt to do so anyway (so-called wall jumpers), and are punished for it, most of the city's citizens never see the other side after the wall goes up.

What's interesting is how this barrier affects the characters and their sense of self. Some people yearn to visit the other side, either because they have family there or because they have the idea that the other side is somehow better. Many characters actually immigrate to the other side. The barrier separates people, but it also makes it impossible for people to have a complete picture of the situation. The very concept of a barrier stokes curiosity about the other side—or in some cases, a sense of superiority among those characters who feel like they're on the better side of the wall—and in other cases, an ineptitude to make a decision about which side is preferable.

"It will take us longer to tear down the Wall in our heads than any wrecking company will need for the Wall we can see," the narrator says, reinforcing this idea that the barrier is not only physical.

A subtheme here is the pain of separation. Many characters in the book are separated from their loved ones when the wall goes up, and are forced to decide whether to immigrate to the other side or to live in the absence of their loved ones. That includes Lena, who is on the opposite side as her family.

Characters on either side of the wall begin to view the other side as being a separate population with its own characteristics. A strange and dangerous phenomenon, it seems, considering that before the wall went up, it was just a big city with many different groups and identities.

The West Berlin side is more modern, while East Berlin is more old-fashioned and has fewer things to buy and eat—just to name one difference between the two sides. Berliners start associating the people on the other side with those qualities, and they're unable to communicate. This furthers a sense of division between the citizens on both sides.