An Extendable Ear consists of a long, flesh-coloured piece of string with a realistic-looking human ear on one end. To use the device, the string is carefully lowered until the ear at its end is positioned near the source of the sound one wishes to overhear, such as outside a closed door. The user holds the other end of the string to their own ear to listen to the captured conversation, which is magically transmitted along the string's length.
The primary magical property of an Extendable Ear is its ability to capture and transmit sound over a distance, effectively serving as a tool for eavesdropping. The magic imbued in the device is sophisticated enough to allow the user to hear clearly through solid obstacles, including thick wooden doors. However, the device is not without its limitations. It can be thwarted by certain defensive enchantments. For example, a door protected by an Imperturbable Charm will prevent the Extendable Ear from working, as the ear will simply hit the invisible barrier and be unable to pick up any sound from the other side. Additionally, perceptive animals like Crookshanks have been shown to notice and interfere with the device, batting it around as if it were a toy.
The Extendable Ears were invented by Fred Weasley and George Weasley before their seventh year at Hogwarts as part of their burgeoning line of joke and trick products for their planned shop, Weasleys' Wizard Wheezes. They were first seen in use during the summer of 1995 at 12 Grimmauld Place, the headquarters of the reconstituted Order of the Phoenix. Harry Potter, Ron Weasley, Hermione Granger, Fred, and George Weasley used them to try and spy on the Order of the Phoenix's secret meetings. Their mother, Molly Weasley, frequently discovered and confiscated the Ears, though the twins always seemed to have a spare supply. One such confiscated Ear was later discovered by Kreacher in a drawing-room cabinet. Later, during their hunt for Lord Voldemort's Horcruxes in 1997, the trio used an Extendable Ear to listen in on conversations at Shell Cottage between Griphook, Bill Weasley, and Mr. Ollivander, gathering crucial information about Goblin beliefs regarding ownership and the history of the Sword of Gryffindor.
The Extendable Ears serve as a key plot device for information gathering, allowing the protagonists to access conversations they are intentionally excluded from. This function is critical at 12 Grimmauld Place, where the adults' secrecy about the Order of the Phoenix's activities fuels the teenagers' desire for knowledge. The invention also serves as a primary example of Fred and George's exceptional magical talent and entrepreneurial spirit, foreshadowing their future success with Weasleys' Wizard Wheezes. Thematically, the Ears highlight the recurring motif of secrecy and the lengths characters will go to uncover the truth.