Shield Charm

"You wouldn't believe how many people, even people who work at the Ministry, can't do a decent Shield Charm."

- George Weasley regarding the charm

The Shield Charm (Protego) is a charm that creates a magical barrier that reflects spells and blocks physical entities. Conjurations may sometimes rebound directly off it back towards the caster, or in other cases, may ricochet off in other directions or dissipate as soon as they hit the shield. There are multiple variations of the Shield Charm.

Casting and effects
When cast, a (usually) invisible shield is conjured in front of the caster's wand, creating a protective barrier between themselves and their attacker. This shield can block and reflect a multitude of spells, such as jinxes and hexes, as well as blocking physical forces. Exceptionally powerful Shield Charms are even able to knock people off their feet. However, this shield isn't completely impenetrable, as it cannot block a Killing Curse.

The difficulty of casting a Shield Charm is relatively moderate. Fred and George Weasley realised that most adult wizards, even those employed by the Ministry of Magic, could not produce a functional Shield Charm (something that not only could Harry Potter accomplish at age fourteen but, also impart to members of the D.A.). They used this to their advantage when marketing their line of protective clothing (hats, cloaks, etc.), upon which permanent Shield Charms were cast.

History
It's unknown who the inventor of the spell was and when it was invented.

In 1484 in the town of Poppleton, the tyrannical Earl of Paunchley held a jousting match. He insisted that all the nobles for miles around must come to cheer him on at the tournament and "no excuse would be acceptable" for missing the event. But young Edmund Gaddlegate fell out of a tree that day and broke his leg, and his mother put him to bed. The Earl wouldn't hear of anyone staying home, so he dragged Edmund and his mother to the castle, and said the mother had to watch while her child was tied to a horse and forced to compete in the jousting.

The other knights left the tournament "in disgust" at the idea of jousting with a small injured boy, so the Earl vowed that he himself would attack Edmund. But a witch in the crowd named Hannah Cockleford had other ideas. Fed up with the evil Earl, she cast an extremely strong Shield Charm between Edmund and the Earl, who was "squashed flat in his armour" when the Shield Charm knocked him backwards and his horse landed on him. The Earl of Paunchley did not die, but when he came-to he was under the impression that he was a donkey named Hairy Cyril.

Harry Potter learned this spell in his fourth year, in preparation for the third task of the Triwizard Tournament. Harry also taught this charm to Dumbledore's Army during the 1995-1996 school year, so the members would be able to protect themselves when the time came. This was also the subject of a question of the Theory of Charms O.W.L. in 1996.

This spell was used frequently during the Second Wizarding War, the Battle of the Department of Mysteries, the Battle of the Astronomy Tower, the Skirmish at Malfoy Manor, the Battle of Hogwarts, as well as on many other occasions.

Known Variations
[[File:Protegomaximahs.gif|thumb|250x250px|Horace Slughorn casting Protego Maxima

]]Other varieties of the Shield Charm include as followed:
 * Protego Duo
 * Protego Horribilis
 * Protego Maxima
 * Protego Totalum
 * Protego Diabolica

Harry Potter
"His Shield Charm was so strong Snape was knocked off-balance and hit a desk."

- Harry Potter's impressive mastery of the charm

Etymology
Latin protego, "I cover" or "I protect".

Behind the scenes

 * In the films, Shield Charms are used in nearly every duel shown, almost always used non-verbally and usually accompanied by a parry-like wand movement.
 * Also in the films, Shield Charms are shown to be able to deflect material objects such as arrows which were shot using a bow. Also it has shown to be able to withstand dragon fire from a Hungarian Horntail as seen in Harry Potter and the Deathly Hallows:Part 1 The Video Game.
 * The book depicts the Shield Charm as only being capable of repelling "minor to moderate curses and hexes", although Harry Potter uses it to block the worst effects of Antonin Dolohov's curse in the Department of Mysteries. However the film implies the Shield Charm to be significantly more powerful in that Harry is able to use it to completely deflect powerful curses cast by Voldemort himself.
 * In the early video games, Expelliarmus functions as a shield charm, due to a lack of enemies to Disarm. In these games, Protego is absent.
 * In the books Protego is also able to keep two quarrelling wizards from going at each other when a shield is cast between them.
 * In, Protego is shown several times to be able to block the Killing Curse. Harry blocks multiple Killing Curses from Voldemort during their final duel and Bellatrix Lestrange blocks four Killing Curses while duelling Molly Weasley. According to the books this is incorrect as the Killing Curse is "unblockable" which is the main reason for its particular infamy, but within the context of the films the fact is never mentioned. As the Killing Curse would instantly end any duel if it were unblockable, this might just be done for dramatic effect and for longer duels. Although it's very probable that this was respectively because of both Harry's mastery of the Elder Wand and Harry's blood being in Voldemorts' veins, and because Bellatrix merely prevented Molly's curses from hitting her with either a Stunning Spell or the Disarming Charm, that they were able to block the Killing Curse.
 * In Harry and Voldemort can use this spell against each other in the final battle during Voldemort's Last Stand as they engage in a normal duel.
 * As Shield Charms can be used as a bewitchment on objects, animals or wizards' clothing to indirectly protect them against many physical and Dark forces, it could be that this way of casting Shield Charms, along with their extended theory, is taught in Charms class.

Notes and references
Schildzauber Charme du Bouclier Encantamiento escudo Kilpiloitsu Protego Щитовые чары pt-br:Feitiço Escudo