Eight Trigrams Sealing Style

The Eight Trigrams Sealing Style, consisting of two Four Symbols Seals, was used by Minato Namikaze to help Naruto Uzumaki access the chakra of Kurama, the tailed beast sealed within him.

The seal was not automatically visible on Naruto's belly, but became perceptible whenever he drew Kurama's chakra.

Whenever Naruto and Kurama conversed, it occurred within Naruto's subconscious, which was depicted as a basement of a boiler room or a sub-level of a power plant, with pipes running across the ceiling and the floor being flooded over with water. On the entrance gate of Kurama's cage, there was a parchment of paper that said, which covered the lock that kept the fox in. Minato also sealed his and Kushina's remaining chakra into Naruto with this seal. Minato appeared in Naruto's subconscious when Naruto transformed into his eight-tailed form, and restored the seal to its original strength, but he could only do it once. Kushina also appeared and assisted Naruto when he later fought Kurama to control its chakra.

Although the seal itself was seemingly destroyed during Naruto and Kurama's fight, it evolved into a new, stronger seal that consisted of massive torii that was capable of imprisoning the fox with ease after Naruto separated it from its chakra and merged with his own. The new seal's gate has the same lock as its previous one.

Key
Because the seal itself would naturally weaken over time, Minato, before his death, created the key, on the abdomen of the scroll toad Gerotora, that can either strengthen or completely remove the seal. He gave it to Jiraiya for safekeeping, with an intention to give it to Naruto by the time he would need Kurama's power to fight against Tobi.