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Harry, after the shopping was completed and everything packed properly, performed one more thing while under the ambient magic of Diagon Alley. He deactivated the Trace. For a normal wizard, this was impossible. The Trace was a difficult-at-best spell to comprehend, a ward that had been weaved into the very fabric of magic, but with his new understanding of magic, Harry was able to unweave this ward from his own magical signature. This was more difficult than he had made it sound when he described it later to Hermione, helping her do the same when they were both at Hogwarts.

Magical signatures weren't confined to a person, but followed a person everywhere they went and being left behind at those places. For the average eleven year old, that meant hundreds of miles of magical signature to unweave the Trace from. The Trace was strongest where the magical signature was strongest, which was why Dobby dropping the pudding had been blamed on him. His magical signature was too strong at Privet Drive to allow the Ministry to pick up Dobby's. This was also why purebloods could get by with doing magic outside of school. The number of magical signatures in the area confused the Trace.

It made sense, when Hermione read about it, that the Trace had been developed by a pureblood and put into law to make it more difficult on Muggleborns. "I'm beginning to wonder if the Americans were right," Hermione would end up saying every time she found something like this.

http://www.fanfiction.net/s/6064546/2/