Eight Trigrams Sealing Style

The Eight Trigrams Sealing Style is a fūinjutsu of the Uzumaki clan.

Overview
A Four Symbols Seal is used to seal a large enemy or an evil spirit into a target, such as an object or a human's body. A stronger seal is created if two Four Symbols Seal are used; this is called the Eight Trigrams Sealing Style. If the Four Symbols Seals do not completely overlap with each other, this allows the enemy/spirit's chakra to merge with whoever they've been sealed into. In the anime, this seal is intended to encourage tailed beasts to aid the jinchūriki they've been sealed within, as the more they provide chakra, the weaker the seal becomes, and thus the better their chance of freedom.

Minato Namikaze used the Eight Trigrams Sealing Style to seal the Nine-Tails into his infant son, Naruto Uzumaki. Before Minato performed the seal, he summoned a ceremonial altar that he placed Naruto on. Naruto's seal is located on his abdomen, but only becomes visible when he uses the Nine-Tails' chakra. As an additional safety measure, Minato also sealed the chakra of himself and Naruto's mother, Kushina Uzumaki, into Naruto, who would appear before him under certain circumstances: Minato appears when the Nine-Tails nearly convinces Naruto to remove the seal, stops him from going through with it, and then reinforces the seal; Kushina appears when Naruto is trying to replace the Eight Trigrams Sealing Style with a new seal, and gives him the support he needs to tame the Nine-Tails.

When Naruto wants to speak to the Nine-Tails, he enters a subconscious plane that resembles an underground complex, with pipes running along the ceiling and various hallways. The floor is flooded with a shallow layer of water; the Nine-Tails' growing influence over Naruto is sometimes depicted as the water level rising. The Nine-Tails itself is separated from the rest of this complex by a large cage; the bars of the cage are far enough apart for Naruto to walk between them, but the Nine-Tails can only fit part of its claws through. The cage's doors are secured by a sheet of parchment with written on it, behind which is the cage's lock. When the Nine-Tails sends its chakra to Naruto, a dark liquid leaks out from the cage towards him.

Because the seal itself would naturally weaken over time, Minato created a key for it that can strengthen seal when needed. He inscribed the key on the abdomen of the scroll toad, Gerotora, and sent it to Jiraiya for safekeeping. Jiraiya, believing that Minato wanted Naruto to use the Nine-Tails against some threat, instead uses the key to slightly weaken the seal, giving Naruto greater access to the Nine-Tails' chakra. After this key is passed to Naruto, he uses it to remove the seal in preparation for replacing it with a stronger one.

Trivia

 * According to Killer B, this seal is of higher quality than the Iron Armour Seal.
 * In the anime, Orochimaru suited Mecha-Naruto with this seal in order to take the Nine-Tails from Naruto.