What are the rules and limits of Accio spell?

Question

We know that Accio spell (aka Summoning Charm) has some limits:

• It can be blocked (e.g. Harry couldn't Accio the Diadem in RoR in Deathly Hallows).

• Many more failures are listed on Wikia.

But what are the exact full rules and limitations governing the spell in Harry Potter world?

• Distance?

• Line-of-sight?

• Knowing exact or approximate location?

• Specificity of item (e.g. "Accio Broom" seemed to work for Weasleys in OotP to bring their locked-up brooms - why not just a random broom?)

• Types of Accio-able stuff? Can you Accio water? Lump of earth? Gases/liquids/etc...?

• Can you Accio based on jQuery selector (or an SQL where clause), for lack of a better description? E.g. "Accio the book that has the word 'Flammel'"? "Accio biggest green ball from this room"?

• Can you Accio ALL items of a kind? (the reason I thought of this whole question was seeing "Accio all guns" comment in this Q&A).

• Can you Accio vague stuff? ("Accio meal").

• Other rules that can be inferred.

_{NOTE: While Wikia article has some of the info, it is clearly very lacking in details on specific rules based on specific instances in canon. I'm looking for a fully comprehensive, organized ruleset for the charm, not merely 1-2 tidbits in Wikia article.}

Answer 1

TL;DR:

• As long as you know exactly what you want, verbalizing the precise name isn't necessary (up to and incluing omitting the subject at all). Ref: GoF, Harry vs. dictionary; GoF, Mrs.Weasley vs WWW products

• No line of sight needed. Ref: OotP, twins vs broomstics.

• No need to know the location Ref: HP, Hermione vs Horcrux books.

• No known distance limits (depends on concentration), Hogwarts to grounds is OK Ref: GoF, Harry vs broom

• SQL query where clause: possible Ref: DH, Hermione vs Horcrux books.

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Specificity of Item: With Accio, you don't always need to know what it is you're summoning, apparently. I mentioned in my answer regarding Hermione and Accio that the first instance of Accio that I could find is Mrs Weasley stripping the twins of their WWW products (in this case, ton-tongue toffees). She doesn't specify what she's Accio-ing but just says "Accio! Accio! Accio!" and confiscates the twins' items. (Goblet of Fire - The Portkey)

Also in Goblet of Fire, Hermione and Harry demonstrate that one doesn't have to exactly name the item being summoned.

‘Well, now we know what to do next time I can’t manage a spell,’ Harry said, throwing a Rune Dictionary back to Hermione, so he could try again, ‘threaten me with a dragon. Right ...’ He raised his wand once more. ‘Accio Dictionary!’

The heavy book soared out of Hermione’s hand, flew across the room, and Harry caught it.

_{Goblet of Fire - page 303 - Bloomsbury - chapter 20, The First Task}

Harry did not say, "Accio Rune Dictionary!" I'm surprised he wasn't pelted by a variety of dictionaries that happened to be around when he practiced this time.

Distance and Line of Sight: When Harry attempts to Accio Hagrid from the ground when Hagrid falls out of the motorbike, he is unsuccessful and later finds Hagrid lying spreadeagled on the ground after the motorbike crashes down from the sky. Harry was not able to see Hagrid when he performed Accio after Hagrid launches himself from the motorbike onto one of the Death Eaters' brooms mid-air and subsequently falls from Harry's sight. (Deathly Hallows - The Seven Potters) As a sidenote, it is possible to Accio living creatures; Harry Accios his toad in Potions in Order of the Phoenix (chapter 18, Dumbledore's Army).

However, Harry Accios his Firebolt from his room in Gryffindor Tower during the First Task of the Triwizard Tournament, sight unseen, from an ample distance. (Goblet of Fire - The First Task) Hermione seems to confirm that distance and line of sight is not an issue as long as the person casting the spell is concentrating fully. Concentration seems to trump distance and line of sight:

‘Harry, I really think you’ve got it!’ said Hermione, delightedly.

‘Just as long as it works tomorrow,’ Harry said. ‘The Firebolt’s going to be much further away than the stuff in here, it’s going to be in the castle, and I’m going to be out there in the grounds ...’

‘That doesn’t matter,’ said Hermione firmly. ‘Just as long as you’re concentrating really, really hard on it, it’ll come.’

_{Goblet of Fire - page 303 - Bloomsbury - chapter 20, The First Task}

Other Actively Magic Items/Accio-ing Any Item: Harry successfully Accios the Triwizard Cup as he's clutching Cedric Diggory's body, while the cup was still activated as a Portkey. I thought it was interesting that Harry merely said, "Accio!" and not "Accio Portkey!" or "Accio cup!" (Goblet of Fire - Priori Incantatum) This reinforces Mrs Weasley's use of Accio under the Specificity of Items section.

On the other hand, Accio doesn't work on Horcruxes, Harry's Invisibility Cloak (which is a Hallow, so presumably Accio might not work on the Resurrection Stone or the Elder Wand), or the Sword of Gryffindor. It seems there are items that are so magically powerful that a simple summoning charm doesn't affect them. In the Horcrux cave where the locket is hidden, Accio causes one of the Inferi to jump from the water, but then it splashes back down into the lake. Dumbledore believed that the Inferi's reaction was intuitive toward anyone who tried to take the Horcrux.

jQuery/SQL where : Yes, it is possible to summon books with a keyword, such as 'Horcrux'.

‘I didn’t think there were any books on Horcruxes in the library?’

‘There weren’t,’ said Hermione, who had turned pink. ‘Dumbledore removed them all, but he – he didn’t destroy them.’

Ron sat up straight, wide-eyed. ‘How in the name of Merlin’s pants have you managed to get your hands on those Horcrux books?’

‘It – it wasn’t stealing!’ said Hermione, looking from Harry to Ron with a kind of desperation. ‘They were still library books, even if Dumbledore had taken them off the shelves. Anyway, if he really didn’t want anyone to get at them, I’m sure he would have made it much harder to –’

‘Get to the point!’ said Ron.

‘Well ... it was easy,’ said Hermione in a small voice. ‘I just did a Summoning Charm. You know – accio. And – they zoomed out of Dumbledore’s study window right into the girls’ dormitory.’

_{Deathly Hallows - page 88 - Bloomsbury - chapter 6, The Ghoul in Pyjamas}

Another example from canon is Harry summoning brooms in Hogsmeade by saying, "Accio Rosmerta's brooms!" (Half-Blood Prince - chapter 27, The Lightning-Struck Tower)

Vague Items: Could a person Accio a "meal"? I think probably. But they may not know what they'll be getting if they merely say, "Accio, meal!" However, if concentration is a big factor, perhaps they could Accio a meal of their choice as long as it's already prepared. Gamp's Five Laws of Elemental Transfiguration specifically states that food cannot be conjured from thin air. It can be conjured if it's already prepared (for example, the food at Hogwarts appearing on the students' plates is conjured (Transfigured) from the kitchens to the plates in the Great Hall). So if a plate of turkey, mashed potatoes, gravy, and green beans is in the kitchen, it can be Accio'ed. If it's not there, then no dice.

So, Accio socks? Sure. They might not be one's favorite pair, though. Accio pillow? It might not be the one you were hoping for (you know...the extra squashy one with the Slytherin pillowcase...)

Accio-able Items: You mention water. The basic spell to summon water is Aguamenti, BUT I think if it were bottled water and one said, "Accio, water!" then the bottled water could be summoned. In Order of the Phoenix, Sirius summons everyone their own bottle of Butterbeer by saying, "Accio, Butterbeer!" Dirt? I don't see why not. Gases? Is a memory in a phial a gas? If it is some sort of gas, then, again, I don't see why not. I think it would depend on the gas in question.

Rules About Accio Inferred: I think the biggest thing to note about Accio is that it is not always necessary to name the item you're summoning as you cast the spell. "Accio!" seems to be sufficient for many, many things. It seems the more dangerous or rare the item, the harder it is to Accio, even impossible in some circumstances (Horcruxes, Hallows, etc.). Otherwise, Accio seems to be a basically straightforward spell.

Answer 2

Wonderbook: Book of Spells lists several rules and limits for accio. It also brings down a story in which those rules aren't exactly followed.

Specificity

Rule
To Summon an object, picture the object clearly in your mind and say ‘Accio’.
Story
Flatworthy did not believe in working, and declared that the only dignified thing to do was to ‘Accio’ more gold. This he attempted to do by pointing his wand in the direction of a nearby goblin workshop. In one sense, his plan worked. Several exquisite pieces of goblin jewellery (as yet unfinished, so without anti-theft charms upon them) came soaring in through the windows of the Accionites’ headquarters.

Food is accioable

Aware that they were still being hunted down by fearsome goblins, and growing both hungry and thirsty, Flatworthy attempted to summon both food and drink to their basement lair.

Animals are useally not accioable

Rule The Summoning Charm is ineffective on most living creatures, and those that can be Summoned are rarely worth Summoning (such as Flobberworms).
Story All we know is that, on the eighteenth of September, 1743, Flatworthy attempted to Summon himself an entire farm complete with livestock, cosy cottage and well- stocked larder. Naturally, the buildings would not shift, but the furious farmer followed his flying cows to the cave on the hill, and discovered Flatworthy, still lying on his cushions, but crushed to death beneath a pile of hay bales and cattle.

Limits

Most magical objects are now sold carrying anti-theft devices that will prevent them being Summoned by any but their owner.

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Once you have mastered this charm, you have the ability to Summon any object of which you have need (providing, of course, that you respect the International Statute of Wizarding Secrecy, since no object should be Summoned if it is likely to whizz under the nose of a Muggle).

Only works on inanimate objects

JK Rowling's new website adds that Accio only directly works on inanimate objects:

Why couldn’t Newt use ‘Accio’ to retrieve all his beasts?
‘Accio’ only works on inanimate objects. While people or creatures may be indirectly moved by ‘Accio-ing’ objects that they are wearing or holding, this carries all kinds of risks because of the likelihood of injury to the person or beast attached to an object travelling at close to the speed of light.

Answer 3

The line-of sight theory is bunk. When Hermione is explaining Accio to Harry, she states that distance is not a factor as long as the image is clear in the persons head. Which would explain how Harry, Fred, and George were able to summon their broom without stating specifically theirs, and how Molly was able to summon the Ton-Tongue Toffees, and Harry summon the cup by just saying Accio.

Answer 4

In Goblet of Fire Chapter 26 we find:

Ron quite liked the idea of using the Summoning Charm again – Harry had explained about Aqua-Lungs, and Ron couldn't see why Harry shouldn't summon one from the nearest Muggle town. Hermione squashed this plan by pointing out that, in the unlikely event that Harry managed to learn how to operate an Aqua-Lung within the set limit of an hour, he was sure to be disqualified for breaking the International Code of Wizarding Society – it was too much to hope that no Muggles would spot an Aqua-Lung zooming across the countryside to Hogwarts.

Assuming that the fact that Hermione didn't object on the grounds that a Summoning Charm wouldn't work indicates that it in fact would work, we can derive from here that distance, line-of-sight, knowledge of specific item, knowledge of location, and specification of item are basically irrelevant.