The Decree for the Reasonable Restriction of Underage Sorcery troubles Harry three times across the books. Note: the reasonable restriction.
Book Two: Dobby drops dessert on Vernon’s clients.
Book Three: Vernon’s sister needles Harry and he spontaneously inflates her.
Book Five: Dementors attack Harry and Dudley. Harry draws his wand and “Expecto Patronum!”
Keep it secret, keep it safe
The Ministry of Magic enforces The Decree when underage wizards perform magic in front of Muggles – Harry’s “crime” in ‘Order of the Phoenix’. The letter from Mafalda Hopkirk*, assistant at the Improper Use of Magic Office, reads: “We have received intelligence that you performed the Patronus Charm… this evening in a Muggle-inhabited area and in the presence of a Muggle.”
Mafalda’s earlier letter, chiding Harry for Dobby’s Hover Charm in ‘Chamber of Secrets’, stated: “Under-age wizards are not permitted to perform spells outside school… remember that any magical activity that risks notice by members of the non-magical community (Muggles) is a serious offence.”
The Decree exists to serve the International Statute of Wizarding Secrecy. Above all else: the magical community should remain secret from the Muggle world. Secrecy is the vital element that must be protected.
Pureblood advantage
Prior to being called up to Hogwarts, Harry unwittingly uses magic in moments of high stress: to regrow his hair, to shrink a detested jumper, to leap away from bullies onto the school roof. Accidental outbursts of magic are indulged for young witches and wizards who are yet to start school. Once they begin at Hogwarts, however, students are expected to control their sorcery in all situations, since they have some training. Hogwarts is subservient to the Statute of Secrecy. The only time Harry disappoints Dumbledore is when he and Ron take the flying car to school in ‘Chamber of Secrets’. “You were seen!” hisses Severus Snape. “Six or seven Muggles in all.”
The Decree for the Reasonable Restriction of Underage Sorcery discriminates against Muggleborns. Hogwarts students from magical families practice magic at home. The Trace, which monitors magic-making in under 17s, does not reveal the spellcaster. Draco Malfoy, aged 16, was tutored at Malfoy Manor by Bellatrix LeStrange. For months before they were of age, Fred and George Weasley developed magical products for Weasley’s Wizard Wheezes at The Burrow. If the Ministry wanted seriously to limit underage magic, as in ‘Order of the Phoenix’, Hogwarts students would store their wands at the school until they turned 17. Dolores Umbridge’s viewpoint is they do not need to defend themselves anyway, so do not need their wands.
Muggle families are treated more harshly than wizarding ones, in the name of the Statute of Secrecy. Muggle families are expected to keep the wizarding world secret, the same as magic families; the Dursleys know Harry is a wizard and know he attends Hogwarts. But because there are no adult wizards in the Muggle households, magic there is flagged.
Magic that matters
What constitutes “magic”? ‘Harry Potter and the Prisoner of Azkaban’ begins with Harry reading under the sheets with a Muggle torch. The movie adaptation depicts Harry practicing the Wand-Lighting Charm under his bed sheets: Lumos, Lumos Maxima. The bright light disturbs Uncle Vernon and Harry feigns sleep. The reason for the change, ob.vi.ous.ly, is to tell the cinema audience: this is the story of an ordinary boy who is a wizard. Efficient visual storytelling.
Lumos under the bedclothes does not break the Decree. Lumos is very light magic (no pun intended): the charm does not leave the wand. There is no action on or change to an external object or being. In ‘Order of the Phoenix’, Harry uses the Lumos spell in a Muggle-inhabited area and in the presence of a Muggle – during the Dementor attack. The Ministry does not add Lumos to the charges against him. The Minister for Magic, Cornelius Fudge, desperately wants to paint Harry as a flagrant rule-breaker. As his case falls apart Fudge brings up Harry’s earlier infractions: Dobby’s dessert, aunt Marge. Yet Fudge does not mention Lumos at the moment when he most wants to smear Harry as arrogant and reckless, as a young wizard who tosses off spells as he pleases. Because reasonably – and reasonably is what counts – Lumos in Little Whingeing does not break the rules.
*Hermione steals Mafalda’s identity for the Trio’s assault on the Ministry in ‘Deathly Hallows’. Karma!