The Lines of Sanzu are a special trait of a shinigami. Represented by 3 lines around the head, it is the key of where a Shinigami manifests its power. How much it surrounds the head is dependent on a shinigami's age. It's been shown that only one shinigami with all three lines connected can exist at a time.
The Lines of Sanzu seem to be directly linked to the Shinigami's own natural special abilities, as Kid was able to access some of his father's abilities after connecting one line. There also appears to be some direct connection to Shinigami himself, as he displays visible signs of pain after the first connection occurred. Spirit's response at this point suggests that the connection of the lines - while technically beneficial to Kid - does not bode well for Shinigami. Shinigami however, sees the lines advancment as progress for his son. Kid immediately concludes that his father gave him Brew because of the power of the Lines of Sanzu, and resolves to meet (what he sees as) his father's expectations. Curiously, he shows no prior knowledge of the Lines' significance, while reacting very quickly to the changes they temporarily brought about.
Later when Death the Kid connected all 3 Lines of Sanzu, Shinigami was killed in the process, crumbled into dust and "absorbed" by Kid, according to Excalibur.. He also learned the true power of a Shinigami, which was to utilize the madness of order and turn people into mindless puppets.
- In Japanese Buddhism, the Sanzu River is the River of Three Crossings and the way to the afterlife. It is the Japanese equivalent of the River Styx. A person's soul crosses from the land of the living to the land of the dead by crossing one of three ways, depending on the weight of their sins, or karma: the good cross a bridge, the neither good nor bad cross through a shallows, and the bad cross through deep waters where vicious snakes swim.
- In the Anime, all three Lines of Sanzu are connected by Kid, which turn Liz and Patty's Death Cannon forms into enormous, golden cannons called the Sanzu Death Cannon. The attack from a single shot from this cannon had the ability to destroy Asura's monstrous form.
- It is possible that, since Kid was born from a fragment of Shinigami's soul, this means they both share one "pool" of power.
- When connected, the Lines of Sanzu resemble a halo, emphasizing how a shinigami is an "Angel of Death."
- In both endings of the anime and manga, when Kid activates all three Lines of Sanzu, it leaves multiple cracks in Shinigami's mask when Kid battles Asura.
- However in the anime, Death doesn't die after Kid activates all three Lines of Sanzu.