Magic Circle

A Magic Circle (魔方陣) is a design or arrangement constructed to facilitate the use of Magic.

Principles
There are various elements which can be used within a magic circle and various forms which they can take - the simplest form is that of a lone circle.

Magic circles often include magical symbols in order to draw power from what they represent via Idol Theory.

Magic circles can be created through various means, from drawing designs and arranging certain objects, to using the positions of the stars themselves.

Index Arc
A unicursal hexagram was one of the elements of the spell which was going to be used to erase Index's memory.

Orsola Aquinas Rescue Arc Arc
During the battle with the Agnese Forces at the Church of Orsola, the Amakusas placed Stiyl Magnus's rune cards in specific positions across the church's grounds in order to form a large magic circle and enhance the power of Innocentius.

Daihasei Festival Arc
During the events of the Daihaseisai and their pursuit of Oriana Thomson, Stiyl Magnus and Tsuchimikado Motoharu explained the basic principles behind magic circles and grimoires to Kamijou Touma.

British Royal Family Arc
During the British Halloween, the Anglicans made use of the special railway car on the underground line below Buckingham Palace, with a magic circle specifically designed as a safety device to stabilize the Curtana in the event of the sword's power going out of control, in conjunction with a flash spell from the Coven Compass to cause Carissa's Curtana Original to overload.

While heading for London on horseback during the coup, Elizard thought about the magic circles in the roads which could have boosted her horse's power and shortened her journey, but being under the jurisdiction of the British government would have alerted Carissa and the Knights to her location if she used them. Later, during the Battle of Buckingham Palace, the captive Bayloupe used a marker pen to draw a communication magic circle to contact the other members of New Light.

World War III Arc
During the later stages of Project Bethlehem, Sasha Kreutzev carved a magic circle to act as a relay point for spells being released from the surface against the Star of Bethlehem, aided by Lessar and the Russian Orthodox magicians onboard the fortress.

Kamisato Rescue Arc
Toyama Luca made use of a unicursal hexagram, drawn through the roads of Academy City with spilled gunpowder while her group were fleeing from the Kamisato Faction and ignited with a musket spark, in the spell used by her and Karasuma Fran to rescue Kamisato Kakeru from the 'new world'.

Necromancer Arc
Numerous magic circles, with elements of both Western Kabbalah and Eastern Taoism, were seen through uses of Rosenthal-style magic during the events surrounding DA, the Hishigata siblings, the Coffins and the Rosenthal Family.

Trivia

 * One particular magic circle is featured prominently in the anime and manga adaptations of Toaru Majutsu no Index, as well as their promotional materials and related media. It consists of a circle containing a pentagram, with a smaller circle circumscribed by the center of the pentagram. Several sets of letters are arranged around the perimeter of the circle, with crosses separating them. In some versions, two larger concentric circles surrounding the base circle and letters. The sequence of letters is as follows (clockwise); "ISCATC+HFIMMMP+MNIP+MNIA+ICY+ISWS".