Patronus Charm

"A Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him. In order for it to work, you need to think of a memory. Not just any memory, a very happy memory, a very powerful memory… Allow it to fill you up... lose yourself in it... then speak the incantation "Expecto Patronum"."

- Remus Lupin teaching Harry Potter the Patronus Charm

The Patronus Charm is a charm that evokes a partially-tangible, positive energy force known as a Patronus (pl. Patronuses ). Patronuses are also called spirit guardians though this may only refer to corporeal Patronuses. It is primarily designed for defence against otherwise unbeatable Dark creatures like Dementors and Lethifolds, though there are other uses.

Casting
"Make it a powerful memory, the happiest you can remember. Allow it to fill you up… Just remember, your Patronus can only protect you as long as you stay focused… Think of the happiest thing you can."

- Harry Potter teaching the Patronus Charm to Dumbledore's Army

To successfully cast the Patronus Charm one must muster a happy memory (the happier the memory, the more powerful and tangible the Patronus will be) and incant "Expecto Patronum". It is unknown if there are any other elements requires in casting, such as physical gestures (aside from drawing and pointing the wand forward prior to or during incantation).

Difficulty
The Patronus Charm is widely regarded as advanced magic (so much so that its not even taught in Charms on the Hogwarts curriculum). It is a very complex charm and many qualified wizards and witches have trouble with it. In fact, Harry Potter is one of the youngest known wizards able to cast a Patronus; he was taught how to do so in 1993 at the age of thirteen by Remus Lupin. In a Dumbledore's Army lesson, Harry taught the members how to use the charm. Some were even successful in casting a corporeal form, though Harry said this might be because there was no Dementor to make them frightened. However, some members were later shown to be able to cast corporeal Patronuses in the presence of Dementors.

Amelia Bones and potential members of the D.A. were all very impressed when Harry Potter revealed that he could cast not only a Patronus Charm but a corporeal one to boot, which is notably more difficult than casting an incorporeal one. Given that the success of the charm is directly reliant on the caster retaining a particular mental state the Charm is more difficult to cast in emotionally trying circumstances. For instance, Harry Potter, Ron Weasley, and Hermione Granger all had difficulty casting their Patronuses when under the negative influence of Salazar Slytherin's Locket, one of Lord Voldemort's Horcruxes, and after witnessing the death of Fred Weasley in battle. This feature of the Charm is particularly unfortunate since the Charm's primary use is to defend against Dementors, Dark creatures specifically equipped to mentally unhinge people (and hence is partly why it is regarded as such advanced, difficult magic).

Self-confidence may also play an important role in casting the spell. In 1994 Harry is able to cast the spell successfully, driving off a large number of Dementors (which according to Severus Snape is only achievable by a very powerful wizard) thus saving Sirius and a younger version of himself, seemingly without recalling a happy memory, when he realizes that he has already done it. Although Harry could have had a "happy memory" upon realizing that he had saved all three of their lives.

The Patronus
A successfully conjured Patronus can take two forms: non-corporeal or corporeal, and both types vary greatly in their appearances and strength.

Non-corporeal
A non-corporeal Patronus is one which does not resemble any living creature and has few distinguishing features (if any). Non-corporeal Patronuses resemble a burst of vapour or smoke without any clearly defined form shooting from the tip of the wand. While they may be partially effective at halting dementors (but not repelling them), non-corporeal Patronuses are not "fully-fledged" Patronuses, and are regarded as a more primitive or weaker version of the true Charm.

Corporeal
A corporeal Patronus is one that is completely and fully formed and takes the shape of a bright-white, translucent animal. A corporeal Patronus takes the form of an animal, like Animagus forms, that reflect the personality of the caster. They are, however, subject to change if the caster goes through an emotional upheaval of some sort. For example, Nymphadora Tonks's Patronus changed from its previous form to that of a four-legged creature which others guessed was a werewolf because then it would reflect her love of Remus Lupin, himself a werewolf. Another example is Severus Snape, whose Patronus was that of a doe, the same as Lily Evans, though this may have been the case all along rather than a change from its original form, as Snape was in love with Lily for most of his life. If a wizard is an Animagus and can summon a corporeal Patronus, the two may take the same form, as was the case for Minerva McGonagall and James Potter. Most Patronuses take the form of an ordinary animal, but magical creatures, such as Albus Dumbledore's phoenix Patronus, are not uncommon. The corporeal patronus is also better at repelling Dementors.

Uses
There are two known uses for the Patronus charm. The first and main use of the Patronus is to drive off certain Dark creatures such as Dementors and Lethifolds, and is the only known charm that will work against them. As Dementors feed on happy memories to render humans to drown in sorrow and sadness, the Patronus acts as a shield of sorts, of which the Dementor would try to feed off of instead - the Patronus is comprised exclusively of positive feelings and cannot comprehend negative emotion, so the Dementor's influence cannot harm it. The other use was devised by Albus Dumbledore, who invented a way of using the Patronus (corporeal and non) as a means of communication. They can deliver messages, speaking with the voice of the caster. This source of communication is believed to be exclusive to the members of the Order of the Phoenix. Using the Patronus for communication offers great advantages in security, as Patronuses identify the caster and are Dark-proof. A good example of this was when Minerva McGonagall used her feline Patronus to call for help to the other heads of houses in 1998 when she found out Harry Potter had returned to Hogwarts in search of Rowena Ravenclaw's Diadem. Another is when Severus Snape used his Patronus to lead Harry Potter to a pool in the Forest of Dean in 1997, where he had laid Godric Gryffindor's Sword for Harry to retrieve.

Etymology
Patronus means "protector", "guardian", or simply "patron" in Latin, specifically reflecting the entire role that the Patronus Charm plays. In archaic Latin, it meant "father", which is very interesting, considering that Harry Potter's Patronus is the same as his father's Patronus and Animagus form, a stag. The Latin word exspecto or expecto means "I look out for" or "I wait for", thus the charm's incantation roughly translates into "I await a protector."

Behind the scenes

 * Both the Patronuses of James Potter and Minerva McGonagall match their respective Animagi form. These are the only characters whose Patronus and Animagus forms are known. It is uncertain whether all Patronuses will match the form of the Animagus, but so far, an instance of differing forms has not been revealed.
 * In the first chapter of Harry Potter and the Deathly Hallows, a white peacock appears in the gardens of Malfoy Manor. Some fans believed this bird to be Lucius Malfoy's Patronus. However, J. K. Rowling stated that no Death Eater except Severus Snape could conjure a Patronus. This means that the peacock was in fact a living creature. It is stated in Harry Potter and the Deathly Hallows that the creature was in fact an albino peacock.
 * In the video game adaptation of Harry Potter and the Prisoner of Azkaban, the more moderate Expecto Patronum is a ball of light (before cast, a white ring goes up Harry's arm and if it reaches top of the wand, Harry has to try the spell again) and is used only to attack Dementors. At the climax of the game, a stag (Harry's Patronus) jumps out of the wand and the player must control it to drive away all the Dementors nearby attacking Harry and Sirius.
 * The lecture that Harry gives on Patronuses to Dumbledore's Army in the film version of Harry Potter and the Order of the Phoenix is extremely similar to the lecture Remus Lupin gave him in the film version of Harry Potter and the Prisoner of Azkaban.
 * In Harry Potter and the Deathly Hallows: Part 1, Dolores Umbridge's Patronus seems to emanate a shield that keeps the Dementors separated from everyone in the courtroom. In addition, Kingsley Shacklebolt's Patronus is described in the novel as taking the form of a lynx, however in the film it is depicted as a shimmering comet of light from which springs silvery images of witches and wizards screaming in terror as Kingsley's voice details the death of Scrimgeour and the fall of the Ministry.
 * In LEGO Harry Potter: Years 1-4, Voldemort, Lucius Malfoy, Walden Macnair, Barty Crouch Jr., and an unnamed Death Eater can all cast the spell, despite the fact that Rowling stated that no Death Eater besides Severus Snape could produce a Patronus.
 * According to W.O.M.B.A.T., it is possible that Patronuses vary in strength according to which animal's form they take.
 * The Patronus spellbook, owned by Remus Lupin, includes information and instruction on the Patronus Charm.
 * In an interview, J. K. Rowling stated that the happy memory that would produce the strongest Patronus for her would be the births of any of her three children, though a close fourth would be when she learned that Harry Potter and the Philosopher's Stone was going to be published. She has also stated that she'd like for her patronus to be an otter, like Hermione's, but she has a feeling it might actually be a large dog.
 * Although Lupin provides most of the information within the books about the Patronus Charm, but his Patronus is never identified.

Patronuses in the films
thumb|350px|right|Patronus being cast.
 * In the film adaptation of Harry Potter and the Order of the Phoenix, his Patronus physically attacks the Dementors by charging at them, as depicted in the books. Also, Patronuses produced by the members of Dumbledore's Army make noises and are capable of interacting with physical objects, such as Ron's dog Patronus knocking over Neville Longbottom.
 * There was a noted change in appearance of Patronuses in general in the Order of the Phoenix movie. In movies 5-8, Patronuses very wispy and airy, while in the third film they looks more substantial, solid and brighter.
 * In Harry Potter and the Deathly Hallows: Part 1, during the infiltration of the Ministry of Magic, Umbridge's Patronus was very clearly seen several times in the same scene and often looked less like a cat and more like a reanimated, feline corpse, with the skeletal features visible. This is odd for a Patronus (they usually looks quite "alive" and well) and its twisted form may be a nod to both Umbridge's nature and the circumstances under which it was cast.
 * In the film adaption of Harry Potter and the Prisoner of Azkaban, the Patronus in its powerful wave-like form emits a deep reverberating sound. In Harry Potter and the Deathly Hallows: Part 2 this effect was also used when Aberforth Dumbledore protected Hogwarts from Dementors.

Appearances

 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 1 (video game)
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * LEGO Harry Potter: Years 1-4
 * LEGO Harry Potter: Years 5-7
 * Harry Potter: Spells

Notes and references
Sortilège du Patronus Incanto Patronus Patronus Патронус Suojelius