Marauder's Map

"Messrs. Moony, Wormtail, Padfoot, and Prongs
Purveyors of Aids to Magical Mischief-Makers
are proud to present
THE MARAUDER'S MAP"

— The Marauder's Map^[src]

The Marauder's Map is a magical document that reveals all of Hogwarts School of Witchcraft and Wizardry. Not only does it show every classroom, every hallway, every corner of the castle, but it also shows every inch of the grounds, as well as all the secret passages that are hidden within its walls. However, people heading out through the secret passage from the Whomping Willow to the Shrieking Shack will disappear, as Snape mentions in Harry Potter and the Prisoner of Azkaban, so the Shrieking Shack is not included on the map because it goes off the edge of the map.

More importantly, the Map shows every single person who is within the boundaries of the Hogwarts grounds as small, black, labeled dots showing, "where they are, what they are doing, every minute of every day", even when the people are transformed animagi, wearing an Invisibility Cloak, or under the influence of the Polyjuice Potion, as demonstrated when Remus Lupin saw Harry Potter, Ron Weasley and Hermione Granger sneaking to Hagrid's house under an Invisibility Cloak.

History

Creation

Marauders1997 — The creators of the Marauders map

The Marauder's Map was created by Remus Lupin (Moony), Peter Pettigrew (Wormtail), Sirius Black (Padfoot), and James Potter (Prongs) while they were attending Hogwarts. The titles on the map refer to these four men in their animagi forms, respectively. This artifact proved quite useful to the foursome (commonly termed Marauders) through the years, though at some point, Argus Filch confiscated it.

Fred and George Weasley

Fred and George Weasley "nicked" it from Filch's office in their first year at Hogwarts, and later gave it to Harry Potter in his third year.

1993-1995

The Marauder's Map with Harry Potter's wand and spectacles — The Marauder's Map in Harry's desk

"Mr.Moony prsents his compliments to Profesor Snape and begs him to keep his abnormally large nose out of other people's business
Mr.Prongs agrees with Mr.Moony and would like to add that Professor Snape is an ugly git
Mr.Padfoot would like to register his astonishment that an idiot like that ever became a Professor
Mr.Wormtail bids Professor Snape good day and advises him to wash his hair,the slimeball"

— The Map's response to Severus Snape's attempt to reveal the Map's secrets in Harry Potter and the Prisoner of Azkaban

Harry uses the Map throughout his time as a student at Hogwarts, especially during his third year, when he tried to find his way into Hogsmeade.

During his fourth year, Harry uses the map to help him as he sneaks into the Prefects' bathroom to work on his clue for the Second Task of the Triwizard Tournament. On his way back, he drops the map and it is almost taken by Severus Snape. Harry is saved by Barty Crouch, Jr., disguised as Professor Mad-Eye Moody who pretended it was his. Crouch later asked to borrow the map from Harry, but after Crouch was unmasked, he reclaimed it.

1996-1998

During Harry's sixth year, when he spent a great deal of time obsessing over Draco Malfoy's daily activities, he used the map often.

When monitoring Draco Malfoy, Harry found he often disappeared off the map. The cause of this was not revealed until after he asked Dobby to trail Draco to find out what he was doing when disappeared off the map. When Dobby told Harry he disappeared into the Room of Requirement, which was not on the Marauder's Map. Whether because it was Unplottable or the marauders never found the room was unspecified.

While on the run in Deathly Hallows Harry would often look at it to see what Ginny was doing.

Use

Marauder's Map 01 — The Marauder's Map,originally a blank piece of parchment.

The map is normally disguised as a blank piece of parchment. To view the map, one must tap it with one's wand and recite, "I solemnly swear that I am up to no good." The content of the map will reveal itself.

To hide the contents of the map so the parchment appears blank again, one must again tap it and recite, "Mischief managed."

If someone tries to view the map without reciting the proper incantation, the map merely insults them and tells them to mind their own business. Severus Snape experienced this phenomenon when he tried to view the map by force.

An added bonus is that if the bearer of the map approaches the entrance to a secret passage protected by a password, the password will appear on the map.

Plot Hole?

The magical defenses on Hogwarts have been mentioned several times, including Hogwarts being unplottable. That means it should be impossible to map Hogwarts, which is exactly what the Marauder's Map does. There are some possible loop holes for this, for example making a place unplottable might simply make it impossible for it to be mapped with reference to somewhere else, while the Marauder's Map is simply a map of Hogwarts, not of its position relative to (for example) London.

Also, it is unusual that a map that normally appears as a blank piece of parchment (as it would have appeared while in Filch's possession all those years) would so easily have let its secrets be uncovered by the twin Weasleys. There is no mention of how they learn to uncover and cover its secrets, something even Snape cannot do when he later caught Harry Potter in the hallways late at night.

However, as it is clear the map still contains some of the personalities of its creators, it is possible that if it finds itself in the hands of the 'right' people it could give them helpful hints as to how to unlock its secrets. For example, when Severus Snape attempted to open the map, the map insulted him using the insults that Moony, Wormtail, Padfoot, and Prongs would have used on him, despite James Potter [a.k.a. Prongs] being dead at the time.

Behind the scenes

There is a toy version of The Map within every Harry Potter and The Order Of The Phoenix Video game which is an exact replica of the in-game Map.
According to J.K. Rowling, Harry never gave the map to any of his children, but James Potter II probably snuck it from Harry's desk.