Unlocking Charm

"There are many ways to pass through locked doors in the magical world (see the Fire-Making Spell, Gouging Spell, and the Reductor Curse) where you wish to enter or depart discreetly, however, the Unlocking Charm is your best friend."

- The introduction to the entry on this spell in Book of Spells.

The Unlocking Charm, also known as the Thief's Friend (Alohomora) is a charm that opens mechanical locks that are not protected by magic. It is also able to open doors locked by Colloportus. There is a spell called the Anti-Alohomora Charm which is used to counteract this spell.

Descriptions of this spell are written in numerous books, including the first volume of the Standard Book of Spells series, where it is described in chapter seven and a spellbook that can be found in the Hogwarts Library.

History
Although it is unknown who invented it (the spell was created in Africa) it was brought to Britain and hence to the world in the 17th century by Eldon Elsrickle. He used it to terrorise London, stealing from homes of Muggles and wizards alike, until Blagdon Blay invented an Anti-Alohomora Charm.

Known practitioners

 * Hermione Granger
 * Ron Weasley
 * Harry Potter
 * Lord Voldemort
 * Miriam Strout
 * Nymphadora Tonks
 * Unknown Death Eater (possibly Lucius Malfoy)

Successful

 * On 31 October 1981 Lord Voldemort used this spell to break into the Potters' house and murder James and Lily Potter.
 * Hermione Granger used this spell on multiple occasions: she used it to open a forbidden entrance on the Third Floor Corridor of Hogwarts Castle in 1991, to open the window to Professor Filius Flitwick's office to rescue Sirius Black in 1994, and to open Regulus Black's bedroom in 12 Grimmauld Place in 1997.
 * The Janus Thickey Ward at St Mungo's Hospital for Magical Maladies and Injuries can only be entered by using this spell. Healers and visitors may enter, but patients are unable to get out.
 * During the Battle of the Department of Mysteries, one of the Death Eaters used this spell to open doors shut by the D.A..

Unsuccessful

 * The door guarding the Philosopher's Stone in 1991 was protected against Alohomora; a winged key was necessary to open it. A single door in the Department of Mysteries was impervious to this spell as well. It is the door which presumably conceals the room where the Ministry of Magic studies the power of Love.
 * In 1996, Harry assumed that Umbridge's office door had been bewitched so that this spell wouldn't work.
 * In 1996, Harry Potter tried to use the spell on the Hogwarts gates with no effect, as Albus Dumbledore had bewitched the gates.

Etymology
J. K. Rowling stated that the word was from the West African Sidiki dialect used in geomancy and has the literal meaning "Friendly to thieves".

Behind the scenes

 * Although Dolores Umbridge's office at Hogwarts was (assumed) impervious to this spell in the novel, Harry Potter uses this spell to open it before attempting to use the Floo Network in the film adaptation of Harry Potter and the Order of the Phoenix.
 * As it was described in a textbook, this may be learned in Hogwarts Charms classes in the first year.
 * This may be the spell that Remus Lupin used to open a wardrobe containing a Boggart, and the spell that Harry Potter used wandlessly to open his cupboard to retrieve his school supplies.
 * In the film adaptation of Harry Potter and the Prisoner of Azkaban, instead of using Alohomora to help Sirius Black escape, Hermione used Bombarda to blast open his cell door.
 * In the Chamber of Secrets video game for GameCube, Xbox and PS2 it's specified that Alohomora is for use on mechanical locks. In most other sources, it's used on magical locks and the spell colloportus as well.
 * In LEGO Harry Potter: Years 1-4 and Years 5-7, Reducto is used to blast open most locks, instead of Alohomora.
 * According to Book of Spells, the spell Portaberto was the most popular unlocking spell before this, and prior to that Open Sesame was.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Philosopher's Stone (video game)
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Goblet of Fire (video game)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * LEGO Harry Potter: Years 1-4
 * Harry Potter: Spells
 * Harry Potter Trading Card Game
 * Pottermore
 * Wonderbook: Book of Spells

Notes and references
Alohomora Alohomora Sortilège de Déverrouillage Алохомора