Undetectable Extension Charm

The Undetectable Extension Charm is a charm placed on any object to make the internal space larger than the external object appears.

Known and Possible Uses
Hermione Granger used this charm on her small beaded handbag in 1997 so that she was able to pack all the supplies she believed she would need while hunting down Lord Voldemort's Horcruxes with Harry Potter and Ron Weasley. This included books, changes of clothes, money, Harry's Cloak of Invisibility, and healing supplies. Eventually, Hermione also placed the portrait of Phineas Nigellus Black that had previously been in 12 Grimmauld Place in her bag as well to prevent Severus Snape from spying on them.

The same, or a similar, spell was used to make Ministry of Magic cars, tents at the Quidditch World Cup, the Knight Bus, and Alastor Moody's trunk larger on the inside.

Behind the scenes

 * The effects of this spell is similar to the effects of a Bag of Holding in many role playing game scenarios and the TARDIS in Doctor Who.

Appearances

 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Deathly Hallows