Trace

The Trace is a charm that tells the Ministry of Magic when magic is performed near anyone under seventeen. The Improper Use of Magic Office learns which spell was cast, where, and when. Underage magic is forbidden by the Decree for the Reasonable Restriction of Underage Sorcery, and the Trace enforces it. Nobody seems to know how a child comes to carry it, though it appears universal among magical children: it breaks by itself on a seventeenth birthday and cannot be placed on an adult. Because it registers magic in the vicinity rather than magic by the child, it is blunt. In a household of witches and wizards the parents' own spellwork drowns out the signal, so the Ministry leaves discipline to them, while a child raised among Muggles is watched far more closely, since any magic near that home is taken to be theirs. At Hogwarts, and on the Hogwarts Express, students cast freely.