Eight Trigrams Sealing Style

The Eight Trigrams Sealing Style, consisting of two Four Symbols Seals, was used by Minato to help Naruto Uzumaki access the chakra of the Nine-Tailed Demon Fox sealed within him. This seal weakens over time. To strengthen the seal again when it gets too weak, Minato drew a key on the abdomen of the scroll toad Gerotora.

When Orochimaru used the Five Elements Seal on Naruto, this created an unbalance due to an odd-numbered seal being placed over an even-numbered one. As a result, Naruto not only lost access to the Nine-Tailed Fox's chakra, but also had difficulty controlling his own, until Jiraiya removed it.