Four elements

The Four Elements (四大属性) are believed to reflect the simplest essential parts and principles of which anything consists or upon which the constitution and fundamental powers of anything are based. In various cultures, religions, and belief systems in the world, the four elements are inexplicably connected to various things, which represent the balance in the world.

In the Toaru Majutsu no Index universe, the elements and their properties are closely connected with Idol Theory, not because it is a requirement in magic, but because of the many legends, myths, and symbolisms that are intertwined with them.

The Classical Elements
There are four classical elements in Western cultures, and if counting the 5th element, quintessence or ether, it matches the 5 elements of Eastern cultures. As such, to Tsuchimikado Motoharu, the Kabbalah, which uses the Western elements, are similar to Onmyoudou, both using the pentagram, one of the fundamental ingredients of a magic circle, which designates the positions of the elements. It can be assumed that the similitarities in the elements in East and West magical disciplines referred to the Japanese Elements.
 * Western Elements
 * Air
 * Fire
 * Water
 * Earth
 * Ether
 * Eastern Elements
 * Wood
 * Fire
 * Water
 * Metal
 * Void

In western magic
Modern Western magic uses, the five elements: Air (or Wind), Fire, Water, Earth, and Ether. Each have their own Symbolic Weapon (象徴武器 (シンボリックウェポン) ); Dagger, Stick, Cup, Disc, and a Lotus Wand which are referred to as the elemental weapons, respectively. Ether has a special characteristic, and that it can change into any of the four remaining elements. It is carried on with the Lotus Wand using Idol Theory, wherein the staff can be made to represent the other elemental weapons.

In fields such as Alchemy, each of the elements has a spirit known as an associated with it.

Directions, colors, and guardians
Accompanying the elements are colors, positions and guardians. These are used in setting up a magic circle, and with accordance with Idol Theory, is used to borrow the powers, abilities, or attributes that are are needed by the spellcaster, these are unique for each one.

Attribution and application
These attributes and its capabilities are explained by Idol Theory, in which a spellcaster would be able to draw power using the symbolic relationship between the elements and its properties. For example: Acqua of the Back has his name referring to his element and the position it has with accordance with Western magic tradition. Along with all members of God's Right Seat, he draws power of the corresponding angel, Archangel Gabriel, this allows him to cast water spells. With color, other than changing the color of clothes, some magicians go a step further, dying their hair, such as Stiyl Magnus, wielding the power of fire, and Aureolus Izzard, drying his hair green, wielding the power of earth. In making magic circles, colors and their corresponding positions is to add decorations that signify its usage, like the All-Around Reality Circle of Tsuchimikado Motoharu using a simple circle that uses the four colors of Eastern Magic to collect information from every single direction.

Chronology
Some time before the beginning of the series, a great distortion of the great laws distorted the elements slightly, and has switched the positions of the angels and their attributes. Despite this, the elements can be still be wielded and called upon with their colors, however, because of this distortion, it allowed a ridiculous spell such as Angel Fall able to be casted.

Angel Fall Arc
With the distortion of the laws of the world, Kamijou Touya accidentally casted Angel Fall, in accordance with Feng shui, arranging the charms, folkcraft, and souvenirs he collected, making his home a ritual location for his spell.

Daihasei Festival Arc
Tsuchimikado Motoharu and Stiyl Magnus explain to Kamijou Touma on how to magicians draw power from magic circles and on how Oriana Thomson's Shorthand spells are similar to Grimoires. Motoharu later sets up a spell in accordance with the teachings of Onmyoudou, that allowed them to track out Oriana's location.

Element Arc
Magic-based cores corresponding to the four elements were used as components for the reduced life forms known as Elements, which Kihara Yuiitsu unleashed on Academy City in order to draw out the Kamisato Faction.