Symbolism, Imagery, Allegory
Throughout this book, Oskar talks obsessively about his grandmother's four skirts. It was under these four skirts, for example, that Oskar's grandfather Joseph fled in order to escape the police. Oskar also claims that Joseph impregnated Anne (Grandma Bronski) while hanging out under these skirts, which led to the birth of Oskar's mother Agnes.
For much of this book, Oskar talks about these skirts as a place of safety, and he feels nostalgic about them, saying
To this day I wish I could lie like a toasty warm brick […] under my grandmother's skirts. (10.8)
Underneath these skirts is a place where everything is simple and safe. Getting under his grandma's skirts might even be a way for him to return to the peacefulness of the womb. His grandmother is an unchanging character in a rapidly changing world—always sheltering and nurturing.