Sonic hat-pin

A sonic hat-pin was a type of sonic device that was used by Paternoster Gang member Madame Vastra. It was a small but ornate device. At its top were a blue feather and a button that could be operated as a remote control for a carriage used by the Paternoster Gang. Vastra occasionally wore the hat-pin on her hat.

In 1890s London, when arriving on Westminster Bridge after a Tyrannosaurus rex had been brutally burned to death in the River Thames nearby, Vastra plucked the sonic hat-pin out of her hat. Pressing the button at the top of the device, Vastra used the device to lock her carriage, which she, Jenny Flint, Commander Strax and Clara Oswald had arrived in but had just exited. The hat-pin caused the carriage to make an electronic chirping noise just before the door on its right side closed. (TV: Deep Breath)

Vastra used the hat-pin to lock Samuel Lumber's Dream Doorway in order prevent any nightmares from coming through the door into reality. (PROSE: The Dangerous Dilemma of the Dream Doorway)

Behind the scenes
The sonic hat-pin was one of three props (all of which were sonic devices used by the Paternoster Gang) that resulted from a Blue Peter competition. The hat-pin was designed by a seven-year-old girl from Kent named Amber. Her design showed that the feather was intended to be from a dinosaur and that the device not only worked as a remote control for Vastra's carriage, but could also transform into a sonic sword that increased in size with the twist of a "volume control" switch. Steven Moffat, who helped judge the competition, commented, "The Sonic Hat Pin is a glorious idea [....] It's funny, it makes absolute sense." He remarked, too, that the small dinosaur's feather "sells it to me." Moffat also mentioned that the sonic hat-pin could "actually summon" Vastra's carriage. The stage directions from the script of Deep Breath referred to the device as simply "the hat-pin," rather than mentioning it being sonic in nature.