jonsnowhite:

The Marauder’s Map is lasting testimony to the advanced magical ability of the four friends who included Harry Potter’s father, godfather and favourite teacher. The map they created during their time at Hogwarts appears to be a blank piece of parchment unless activated by the phrase: I solemnly swear that I am up to no good, a phrase that, in the case of three of the four makers, should be understood as a joke. The ‘no good’ of which they wrote never denoted Dark magic, but school rule-breaking; similar bravado is evinced by their use of their own nicknames on the map (‘Messrs Moony, Wormtail, Padfoot and Prongs’).

The magic used in the map’s creation is advanced and impressive; it includes the Homonculous Charm, enabling the possessor of the map to track the movements of every person in the castle, and it was also enchanted to forever repel (as insultingly as possible) the curiosity of their nemesis, Severus Snape.