A Circle helps a wizard refine his magic, focus and direct it more clearly. It creates a sort of screen; defined by a perimeter keeping random magical energy from going past it. It can be drawn on the ground, made by several people holding hands, or a number of other methods. Then it's invested with a bit of energy to close the circuit.
Draw a circle, then put a drop of own blood on the circle, and imagine a wall going up. Chalk works but various things can be used to make the circle itself: marker, pouring dirt or powder, etc.
Walking slowly clockwise around a circle fashion is called deosil and is done to gradually power up the circle.
Circles create boundaries that isolate an area inside from the magical energies of the world outside.
Circles are an effective and standard way to trap a spirit, making it helpless to leave or use power. A circle made of fire can destroy the spirit.
A Greater Circle is a very powerful form of a circle used to contain a person or being with incredibly powerful magic, such as the Archive. It requires an enormous amount of steady, powerful energy. Hellfire is powerful but very unstable, flowing in starts and sputters. To hold a Greater Circle of long periods would require the kind of power found in ley liness.
Learning how to properly establish a Circle is the first thing any apprentice is taught.
The integrity of a Circle as part of ritual magic is most important. Any object that falls across it or breaks its plane would collapse the circle energy. Dust wouldn't collapse it but it would degrade its efficiency.
In the seriesEdit
In Storm Front, Dresden puts himself and Susan Rodriguez inside his brass circle in his lab and wills it closed, just in time before the Toad Demon spits acid at him which splatters against the circle's barrier. Susan had just taken a love potion by mistake and it starts to take effect while they are inside the circle, and one wrong move will break the circle. 
In Fool Moon, Dresden is approached for information about greater Circles by Kim Delaney. He refuses to provide any substantive details, on the grounds that the circle she's asking about is one that would only be used to hold extremely powerful supernatural beings well beyond her ability to control. She attempts the Circle anyway, an attempted replacement for a similar, but now ruined, one used by Harley MacFinn to contain himself during his loup-garou transformation, dying when the monster smashed its way through her flawed Circle and ripped her to pieces.
In Grave Peril, in his lab's copper Circle, Harry Dresden attempts to summon Azorthragal who was the Demon that Leonid Kravos made efforts to secure as his servant. Dresden ends up summoning the Nightmare instead, assuming Azorthragal is the Nightmare.
In Blood Rites, Harry Dresden enlisted Jake Guffie to help with setting up circle around Arturo Genosa's studio to block the Malocchio curse. Jake assumes Dresden is doing some sort of feng shui, and Dresden agrees that it is. Later, Trixie Vixen told Dresden that she had watched him make it and then broke the Circle when he wasn't looking.
In Dead Beat, Harry Dresden told Waldo Butters that a Circle can stop zombies by cutting off the flow of energy from the necromancer controlling them. Dresden taught Butters how to make a Circle around himself to block the magic emanating from the Cowl's storm making so that he could power-up the GPS system they plan to use to locate the Word of Kemmler]]. During a Specter attack near the Darkhallow site, Butters empowered a circle around himself, protected from the Specters, he kept up a steady beat for Sue.
In Proven Guilty, with seconds left of consciousness, Harry Dresden drew a circle in the dirt around himself while he was being choked by the severed hand of the Scarecrow, cutting off the power connection to the Scarecrow and allowing him to breathe again. The hand turned to Ectoplasm.
In "Day Off", Harry Dresden chalks a circle around Kirby and Andi Macklin which cuts off the cut the connection between them and the Psychic parasites in the Nevernever, returning them to sanity, for a while.
In Turn Coat, Harry Dresden draws a circle around Murphy and himself when they are surrounded by Grey Suits. The repeatedly attempt to enter the circle by bumping up against it then by pressing their palms—which had mouths containing sharlklike teeth, purplish-black tongues, and yellow mucus—against the circle. Then, Molly Carpenter draws a circle around Binder's minions, the Grey Suits, at Donald Morgan's suggestion, which cuts off the flow of energy they needed to maintain solid forms. They suddenly melted into transparent ectoplasmic gook. 
In Changes, Harry Dresden activates his Speaking stone, (aka Sending Stone) to ask Ebenezar for help from the Grey Council is locating where the Reds took Maggie, his daughter. Ebenezar cuts Dresden off before he can tell the child is his daughter and scolds Dresden for thinking only of one child and not the millions of children that the White Council protects from the Reds.
After blanking out with Ebenezar McCoy, Dresden summons a multitude of nature spirits, water spirits, and messenger spirits from the various realms in the Nevernever. And servants of the Tylwyth Teg whose king Dresden is on good terms with. Even a sentient flame creature who communicated in images. One of the fire spirits showed Dresden an image of Maggie shivering in fear or cold, reaching out for a flame for warmth. Later, while Dresden was lying with a broken back in Saint Mary of the Angels alone in the dark, he forms a Circle in his mind to summon Uriel and then Mab.
In Cold Days, Harry Dresden seals a Circle of black pepper he poured from a shaker around Thomas's phone. Then he hexes all of the electronics in Thomas's apartment—the phone is protected from the hex to remain usable—then Dresden breaks the Circle. This spell hexed all of the bugs that Lara Raith regularly places around Thomas's apartment. Dresden's ulterior purpose to hexing the bugs was to alert Donar Vadderung as to how Lara operates when her operatives follow-up on Dresden's call to him. Dresden calls it a "friendly gesture", Thomas suggests it was a payment for meeting him.
The Ladies of Faerie, Lily and Maeve, made an enormous powerful circle that tapped into Demonreach's basic architecture. Only Dresden was able to enter and he had to do so naked. The Ladies were attempting to wear down the defenses of the Demonreach Entity to release what is underneath it. Dresden's friends figured out how to pass through the wall of the circle—by covering themselves with mud, they fooled it into identifying them as part of the island and so ignoring them. Dresden used that Circle to summon Mab. 
- ↑ 1.0 1.1 Blood Rites, ch. 25
- ↑ 2.0 2.1 Storm Front, ch. 6
- ↑ 3.0 3.1 Dead Beat, ch. 36
- ↑ 4.0 4.1 Changes, ch. 29
- ↑ Storm Front, ch. 8
- ↑ Fool Moon, ch. 7
- ↑ Changes, ch. 9
- ↑ Ghost Story, ch. 11
- ↑ 9.0 9.1 9.2 Turn Coat, ch. 19
- ↑ 10.0 10.1 Ghost Story, ch. 15
- ↑ 11.0 11.1 Dead Beat, ch. 6
- ↑ Small Favor, ch. 39
- ↑ 13.0 13.1 13.2 Changes, ch. 19
- ↑ Storm Front, ch. 13-14
- ↑ Fool Moon, ch. 1
- ↑ Fool Moon, ch. 12
- ↑ Grave Peril, ch. 23
- ↑ Summer Knight, ch. 19
- ↑ Blood Rites, ch. 24
- ↑ Dead Beat, ch. 39
- ↑ Proven Guilty, ch. 22
- ↑ Small Favor, ch. 42
- ↑ Changes, ch. 30
- ↑ Cold Days, ch. 20
- ↑ Cold Days, ch. 46
- ↑ Cold Days, ch. 50
- ↑ Cold Days, ch. 51