Grimmauld Place

Number twelve, Grimmauld Place, London is a fictitious
house in the
Harry Potter books. It is the
ancestral home of the the Black family, an old-established
wizarding family, and first appears in Harry Potter and
the Order of the Phoenix.

"Grimmauld Place" is a pun on "grim old place".

Spoiler warning: Plot and/or
ending details follow.

Harry Potter Locale
Number 12, Grimmauld Place
Location	London
Affiliation	Order of the Phoenix
Permanent Residents	Kreacher, Sirius Black (until he is killed)
First Appearance	Harry Potter and the Order of the Phoenix

Grimmauld Place was the ancestral home of The Black family. It eventually passed into
the hands of Sirius Black, the "white sheep" of the Black Family and last
remaining member of a line of traditionally very dark wizards. The
house promptly fell into disrepair as its owner was locked up in
Azkaban
prison. The result was a gloomy and unpleasant dwelling teeming
with dust, decay and various malevolent objects, pests, and hexes. During this
period, inhabitants and "inhabitants" included a hysterical
portrait of Sirius Black's mother who had
a deep-seated hatred for non-Purebloods,
an ancient and deeply mad house-elf named Kreacher who heartily seconded this opinion and a
portrait of Phineas Nigellus, a former headmaster of
Hogwarts.

There were many security measures in place at Number 12 -- for
example, in order to disguise the house from muggles and other
interlopers, it was enchanted to appear small and modest from outside, though its
interior was actually large and opulent. For this reason, it
was chosen as the headquarters of the reorganized Order of the Phoenix. Thereafter, its
location was further hidden through the use of the Fidelius Charm with Albus Dumbledore as the Secret Keeper.
Dumbledore used the portrait of Phineas Nigellus for communication
with the Order, as a second portrait was hung in the headmaster's
office at Hogwarts. During the period of Harry Potter and
the Order of the Phoenix, many members of the Order
frequently came and went. Harry
Potter's only two visits to the house were during this period
as well, in August and December of his fifth year at Hogwarts.

Sirius Black was forced to stay at the house of
his ancestors after his escape from
Azkaban because the Ministry of Magic was still on the lookout
for him. It was quite possibly Severus
Snape's taunting that drove Black to leave the house on the
night where Harry and several members of the Dumbledore's Army, or DA, went to the
Ministry of Magic. For more information, see the fifth
book.

Harry inherited the house at the beginning of Harry Potter and the
Half-Blood Prince. Dumbledore had removed the Order of the
Phoenix until he could be sure that Harry really owned it. The
spells put in place by Sirius' family might have ensured that their
home could only pass to family members or to pureblood wizards, in
which case it would have belonged to Sirius' closest living
relative (and his murderer): Bellatrix Lestrange. Dumbledore tested
Harry's ownership of the house by having Harry give orders to
Kreacher, the house-elf servant, reasoning that if
Harry had truly inherited Kreacher that would indicate that he had
truly inherited the house as well. Kreacher did follow Harry's
orders, however reluctantly, so it was clear that Harry did in fact
own Number 12, Grimmauld Place.

It is suspected that a locket found during the cleanup of the
place in book 5 is one of Voldemort's Horcruxes,
Slytherin's locket, which was found by a
person with the initials R.A.B. When the trinket was uncovered, no one
could open it. It is thought that he is probably Sirius' brother
Regulus Black. If so, the locket may still be
in the house, may have been thrown out with the rubbish, may have
been stolen by Mundungus Fletcher or may have been
hidden by Kreacher.

As revealed by J. K. Rowling on her website on February
21 2006:

"When a Secret-Keeper dies, their secret dies with them, or, to
put it another way, the status of their secret will remain as it
was at the moment of their death. Everybody in whom they confided
will continue to know the hidden information, but nobody
else."

Thus, it is conclusive, that nobody whom Dumbledore did not
invite inside will be able to enter.