A bell jar is a glass, open-bottomed container which is typically placed over objects to enclose them in an airtight space. This confining container is the novel’s titular symbol.

Throughout The Bell Jar, Esther visualizes her mental illness as a bell jar that descends and traps her inside. From within its confines, she can see the outside world, distorted through the glass, but she cannot engage with it. Like the imagined bell jar, her illness warps her sense of perception, turning ordinary things into threats and warping her sense of identity. The bell jar cuts her off from everyone and everything, trapping her with her own “sour” thoughts, a reflection of her agonized mental state. Esther cannot connect with her loved ones or friends from inside the jar, and since they don’t understand her mental state they cannot come into the jar with her. The visual of being trapped under a glass dome highlights the profound loneliness of Esther’s condition.

Bell jars are often used to put objects on display. In the asylum, Esther feels like she is on display to her doctors and visitors. They view her like a fascinating oddity, like the preserved fetuses from Buddy’s medical school, and she is unable to get through to any of them just as they cannot get through to her.