Zanpakutō

A  is the main weapon of the shinigami and the arrancar. The art of using a zanpakutō is called zanjutsu (swordsmanship).

Overview
Most zanpakutō look like regular katana, with slight variations between them as one would expect between different swords. Each zanpakutō is imbued with a unique sentient spirit, which gives the sword the power to send pluses to the afterlife, purify hollows, and transform the zanpakutō itself into more elaborate and powerful forms. To facilitate these tasks, zanpakutō have unique abilities that match their owner's characteristics.

A zanpakutō spirit is part of its owner's soul, and often share the same personality traits. At the same time, they are also independent, and thus can be considered a shinigami's partner in battle. Because they are part of their owner's soul, a zanpakutō cannot be replaced, though it will slowly regenerate if broken. As a shinigami gains power, their zanpakutō can change form by allowing the inner being to manifest itself to a greater degree in the outside world. The sword and the inner being are synonymous to each other. No matter what form (or size) a zanpakutō takes, it is always virtually effortless for its owner to wield because it is a part of its owner's soul. On the other hand, it is much more difficult for a shinigami to wield a zanpakutō that is not their own; Kaname Tōsen is the only character in the series to actually do such a thing.

Forms
Depending upon the shinigami's ability to communicate with and control their zanpakutō ' s spirit, a zanpakutō can manifest itself in two additional forms and revert to a sealed state. These two forms, known as shikai and bankai, are akin to "upgrades" for the zanpakutō, giving it abilities far beyond that of its simple use as a sword. Shinigami usually carry their zanpakutō in the sealed state and activate the released forms as necessary. Shinigami with zanpakutō of the constant-release type, such as Ichigo Kurosaki, are unable to seal their zanpakutō and therefore carry them in released form constantly.

In addition to the upgraded forms, the sealed form of a zanpakutō can also be radically altered, though such alterations are purely cosmetic and usually don't offer any significant advantage in battle; one exception is Ikkaku Madarame's Hōzukimaru, whose hilt stores a blood-clotting ointment. Shinigami with great spiritual power and who are unable to control it have larger zanpakutō, as Ichigo Kurosaki did when he first became a shinigami. Isshin Kurosaki, Ichigo's father, explains at one point that high-level shinigami must be capable of maintaining and suppressing the size of their sword. Otherwise, they would end up swinging around swords the size of skyscrapers. Zanpakutō can also be disguised as other objects, though such objects seem to need to be of similar proportions to the sword itself. Both Kisuke Urahara and General Yamamoto, for example, disguise their zanpakutō as canes.

Shinigami who are unable to communicate with their zanpakutō wield a nameless version known as asauchi.

Shikai
The is the first "upgraded" form available to a zanpakutō. To activate it, the shinigami needs to learn the name of their zanpakutō. This is not as simple as simply picking a name, as the living spirit of the zanpakutō already has its own name. Therefore, the shinigami must be able to communicate with their zanpakutō effectively, which requires being able to speak to the spirit within its world. Achievement of the shikai is a mark of control of a zanpakutō, and it appears to be a requirement for advancement within the shinigami ranks, as most (with the notable exception of Kenpachi Zaraki) seated officers and presumably all lieutenants are capable of shikai.

After first learning the zanpakutō ' s name, the shikai can be performed at will simply by speaking a command followed by the zanpakutō ' s name. The command varies between users and range from a simple imperative verb to a short poem. It often relates to the zanpakutō ' s signature ability, or hints at the nature of its spirit. This step may be bypassed by expert shinigami who have learned how to use their bankai. In rare cases, a zanpakutō, once released, will remain in that state indefinitely. Such zanpakutō are described as a full-time released form type, such as Ichigo's.

Once the shikai is activated, the zanpakutō will change form to access whatever special abilities it is capable of. They usually remain in the form of a sword (With, so far, the exception of Soifon wielding a stinger, Ikkaku Madarame wielding a spear, Kaien Shiba wielding a trident and Marechiyo Ōmaeda wielding a spiked ball), but the type of sword differs radically among various zanpakutō. The special abilities of the zanpakutō, like the zanpakutō itself, have specific names that usually must be announced to fully activate the effect. As Ichigo's zanpakutō, Zangetsu, explains, knowing the name of an attack and not knowing it makes a difference in the amount of power that attack has.

Bankai
The is the second and final form of a zanpakutō. To achieve it, one must be able to materialize their zanpakutō ' s spirit in the real world and subjugate it, giving the shinigami full reign over the zanpakutō ' s power. According to Yoruichi Shihōin, the power of bankai typically increases a shinigami ' s power by a factor of ten. Because such an act usually takes hundreds of years of combat experience to achieve, it is rare amongst shinigami and almost exclusively limited to the thirteen shinigami captains in the Gotei 13. Byakuya Kuchiki claims that among the four noble houses of Soul Society, only one person in several generations will be born with the spiritual power necessary to achieve it. Ichigo Kurosaki and Kisuke Urahara are exceptions to this fact. Using a special device created by Urahara, which forcibly materializes a zanpakutō spirit (thus bypassing the experience portion of the training), they were able to attain their bankai within three days instead of the normal hundreds of years. Renji Abarai is also an exception ,but he did not use Urahara's device. The other exceptions are Ikkaku Madarame, who was able to perform bankai from his own years of training, and Tōshirō Hitsugaya, the youngest ever shinigami depicted so far to attain "bankai".

After a shinigami is able to materialize and subjugate their zanpakutō ' s spirit, the zanpakutō can assume its final and most powerful form. Unlike the shikai, there is no command phrase to activate bankai, though the user will usually say "bankai" beforehand. A zanpakutō ' s bankai is often a manifestation of the spirit itself, usually creating a massive creature or effect to aid the user in battle (Ichigo's bankai is an exception, becoming more compact instead). The bankai itself will receive a different name from the ordinary zanpakuto (Ichigo's Zangetsu becomes Tensa Zangetsu, Byakuya's Senbonzakura becomes Senbonzakura Kageyoshi). Certain shinigami also change with their zanpakutō, gaining clothing similar to what their spirit form looks like. Most zanpakutō will also gain additional special abilities. Like with the shikai, most of these abilities have an associated name. Maintaining the bankai is taxing for the wielder, and it is difficult to maintain it for long periods of time.

Types
There are several types of zanpakutō, classed by the main purpose or effect of their special abilities. For example, Ichigo Kurosaki's zanpakutō is a melee-type zanpakutō, since it is primarily designed for close-combat; though it could also funtion as range-type thanks to its Getsuga Tensho ability. Yumichika Ayasegawa's zanpakutō, on the other hand, is a kidō-type zanpakutō, since it's designed around its special ability rather than its close combat potential.

Some zanpakutō are also classified by element: Tōshirō Hitsugaya and Rukia Kuchiki possess ice-type zanpakutō, Shigekuni Yamamoto-Genryūsai controls a fire-type one, and Kaien Shiba's uses water-based abilities. Wielders of Elemental-type zanpakutō are also notable to possess a certain immunity against the very elements that their weapons control

Another type of zanpakutō, known as the constant release-type. Actually a 'sub-type" of zanpakutō classification, this group pertains to zanpakutō whose wielders are unable to revert them back to their default sealed states after initiating the Initial Release. Such a phenomenon is mainly due to the vast spiritual power of the sword's wielder that they are unable to completely seal their zanpakutō like any other Shinigami. Indeed, only two known shinigami are known to wield the constant release-type: Ichigo Kurosaki and Kenpachi Zaraki, both of which have tremendous spiritual power exceeding that of a normal Captain-class Shinigami.

Though not mentioned, it does seem that bankai makes the effect of the shikai special attacks stronger. The power of Ichigo's Getsuga Tenshō, for example, increases when bankai is used.