When did the "trace" come about?

[abeandamandabustos]

No: HPFGUIDX 176137

In the previous books the decree for the restriction of underage magic
is only referred to as such and is never said to be a "trace."  DD even
states in HBP that the MOM cannot even differentiate between who is
performing magic in any given place. Harry was blamed for Dobby's
hover charm because the MOM did not know who cast the spell, simply
that a spell had been cast.  Now in DH, there is a trace that knows who
does what and Harry cannot perform magic at the Burrow- even though
previous canon (via DD) states that the MOM relies on wizarding
parents to restrict thier children. Any thoughts?

Amanda- first post- hope it didnt suck!