Muggles' Guide to Harry Potter/Places/Knockturn Alley

General Overview
Knockturn Alley is a street of shops and vendors specializing in the darker forms of magic. The one shop named on this street is Borgin and Burkes.

Extended Description
As there is a part of London where regular wizards can go to buy magical equipment, namely Diagon Alley, so must there be a place where dark wizards carry out their business. Knockturn Alley (pun: "nocturnally") is that place. Shunned by all right-thinking wizards, Knockturn Alley is the place where you would go to buy a Hand of Glory, Flesh-Eating Slug Repellent, or "things that looked alarmingly like whole human fingernails". It holds a certain illicit glamour for young wizards, like Fred and George Weasley, who (once they set up shop) likely use certain ingredients available only there for some of the magic that they sell in their store.

In Harry Potter and the Chamber of Secrets, when Harry is using Floo powder to get to Diagon Alley, he apparently travels "one grate too far", accidentally turning up in Borgin and Burkes, on Knockturn Alley. He is led out of Knockturn Alley and back to Diagon Alley by Hagrid, who had gone to get some flesh-eating slug repellant. Later, when the party breaks up to do their separate shopping errands, Mrs. Weasley feels she has to warn the twins not to go into Knockturn Alley.

At one point when the Weasley family are preparing to visit Diagon Alley to buy school supplies, Mrs. Weasley again explicitly warns the Twins not to enter Knockturn Alley.

In Harry Potter and the Half-Blood Prince, Harry, Hermione, and Ron sneak off from Weasleys' Wizard Wheezes and follow Draco Malfoy into Knockturn Alley. They then eavesdrop on him as he talks to Borgin about something he is buying, using Extendable Ears. Hermione's subsequent attempt to determine what it was that Draco was interested in fails, as she is not good at that kind of subterfuge.

Analysis
Many readers must wonder why such a hot-bed of Dark magic is allowed to remain so close to the main mercantile center of Wizarding London. Would it not be better to eliminate it and the associated Dark enterprises? Likely it has not been cleaned up for the same reason that spymasters are allowed to stay in business when they are known &mdash; if you keep all your evil-doers in one place, it is much easier to keep track of them.

Greater Picture
Though we only enter Knockturn Alley twice, we visit Diagon Alley in five of the seven books. (Mrs. Weasley does the school shopping in Harry Potter and the Goblet of Fire and Harry Potter and the Order of the Phoenix.) With the ascendancy of Voldemort, Diagon Alley gets successively less cheerful. By the time of our visit in Harry Potter and the Half-Blood Prince, there seems to be little difference between Diagon Alley and Knockturn Alley, and by the time of our visit in Harry Potter and the Deathly Hallows, the only difference seems to be that there are more shuttered or boarded up places of business in Diagon Alley.