Death Note has a well-deserved reputation as one of the greatest thrillers in the anime canon. The wars of inductive reasoning between star student Light Yagami and world-class detective L are unlike anything before or since, but it's not just the cat-and-mouse dynamic that makes the show great. Light and L live in a shockingly detailed fictional world with intricate rules to its supernatural elements. Here are just ten of the rules of the Death Note, as seen in the anime and expanded upon in the thirteenth manga volume, How to Read. These are some of the most important guidelines to the story and setting.
I.1. The human whose name is written in this note shall die.
Cultural osmosis. Even if you haven't seen Death Note, odds are that if you're familiar with anime at all, you know this line. It's one of the few certainties in the series, and one that every other event and decision revolves around. Think of the plot of Death Note like a Superman comic book: with a hero that's unstoppable, the suspense revolves around preventing him from exercising his power.
Of course, killing someone isn't as easy as writing their name. Even before he uses the Death Note for the first time, Light is well aware of the potential repercussions. He spends a good chunk of the first episode mulling over his choice for a first victim, concluding that it cannot be someone he knows. It would attract too much suspicion to kill even a passing acquaintance.
That's the world Light lives in: every action has unforeseen ripples, and he soon finds himself up against adversaries that know exactly how to read them. It's possible to manipulate the conditions of death beyond a simple heart attack; paradoxically, due to the prowess of Light's foes, such elaborate measures are actually easier in the long run. Light's experiments with the Death Note's capabilities begin in the first season and never stop, as a matter of curiosity—and of survival.
(Note: the term "note" as used in the rule means "notebook." "Death Note" and "Death Notebook" mean the same thing.)
I.2. This note will not take effect unless the writer has the person's face in their mind when writing his/her name. Therefore, people sharing the same name will not be affected.
The second rule is even more important, as it makes the first possible. In purely mechanical terms this rule makes sense—you can only kill one person at a time. But it's also the only defense against the Death Note: one must either remain hidden or use an alias. For a professional undercover detective like L, this is simple.
But figuring out someone's identity is only one step. How do you use that information without incriminating yourself? This is why it takes so long for L to finally die, even when Misa Amane knows his name from the moment they meet. When L tells the police to assume Misa is Kira if he dies, he's weaponizing his proximity to her and Light. He's invincible even if Light and Misa know his name and face, because as prime suspects they cannot act on that knowledge.
Killing a detective you work closely with is fundamentally different from killing a convict who appears on national news. Knowing an identity may make it possible to kill someone, but it's another thing entirely to take a life without revealing that you know something you shouldn't.
II.3. The human who uses the notebook can neither go to Heaven nor Hell.
This one's a bit of a red herring. The second to last rule of the Death Note states that "After [humans] die, the place they go to is MU (nothingness)." So why mention Heaven or Hell if they don't exist? Is it a deterrent for all but the most sociopathic humans—the "interesting" ones, as Ryuk would put it?
Perhaps. Perhaps it also refers to the fundamental immortality of a Death Note user in the minds of those who knew them. Consider Light's impact on the world in the five years he has the Death Note. The world is gripped by the fear of an unknown god who kills as he pleases. On the other hand, all wars have ground to a halt, and that fear has created a seemingly better existence for humanity. The debate rages from beginning to end, in the general public and among the ranks of the Kira task force: Is Kira good? Is he evil? Is he somewhere in between?
Even after Light's death, pockets of Kira worshipers remain, waiting for their god to deliver them from the wicked—or perhaps the responsibility of deciding for oneself who is wicked. In a world where Heaven and Hell are proven to be human constructs, perhaps this rule simply speaks to a duality in all people. We as a species cannot decide whether Kira is a god or a demon. This rule is one of the first and most well known, and the fact that its meaning so directly contradicts the reality of its world speaks to our inability—or refusal—to answer the questions that make us human.
IV.4. A god of death has no obligation to completely explain how to use the note or rules which will apply to the human who owns it unless asked.
I mentioned certainties before, and this is another one. In the very first episode, Ryuk outright states that he will one day kill Light. He only follows Light around because he's bored. He doesn't have to explain the Death Note to Light at all. But note the last two words: "unless asked." Although Ryuk skirts the Shinigami laws to get the series rolling, he is still bound by them in various situations.
The gods of death exist to kill humans. It's how they remain alive. There's no escaping that, and none of them are particularly interested in assisting humans on their own initiative. But their interactions with mankind are also highly regulated, and the nature of their relationship to us is unclear, as are most other things about them. Why are they all so lazy? Why do some of them just forget to write names? And why, in Ryuk's case, are they required to provide information when asked?
Shinigami have the power to omit or to evade, but not to lie, which raises all kinds of questions about how we deal with death. But make no mistake: it falls to humans to ask the right questions.
V.3. The human who becomes the owner of the Death Note can, in exchange of half his/her remaining life, get the eyeballs of the god of death which will enable him/her to see a human's name and remaining life span when looking through them.
How far are you willing to go to fulfill your vision, and at what cost? This is one of the few lines that Light decides he cannot cross—not out of any particular necessity, but simply because he would rather not cross it. More than any other rule in the series, this one serves as a litmus test for the character of a user of the Death Note. Light will not shorten his own life for personal gain, but is perfectly willing to ask others to do it for him. Misa does it because she loves Light, but hers is an obsessive love that destroys everything in its path—her most of all, as she makes the deal three times.
If there's a case to be made for a "correct" use of the eye power, then Soichiro Yagami makes it. His decision to make the deal is a selfless sacrifice in the name of bringing a killer to justice, and not one he makes lightly. Light's father is alone among the cast in accepting ultimate power with the intent that it never be used again.
VI.1. The conditions for death will not be realized unless it is physically possible for that human or it is reasonably assumed to be carried out by that human.
Light learns this one early on during his experiments with convicts. It's the reason you can't have a prisoner in Japan die at the base of the Eiffel Tower. This rule is an important constraint on the circumstances of death as ordered by the Death Note, and it's also an oblique commentary on human nature. Another rule, for example, explicitly mentions that "all humans are thought to possess the possibility to commit suicide."
Although each death is caused by the Death Note, it spurs speculation about what the average human is capable of. Though the Death Note is generally restricted to manipulating circumstances, in certain situations—particularly where suicide is concerned—it is shown to affect the psyche of a victim. The criminal that hijacks Light's bus may have done something similar anyway, but what do we make of the strong-willed Naomi Misora's sudden decision to kill herself? What was she exposed to that made life so irretrievably pointless for her? If some fundamental force exists that can drive one to unspeakable acts, what might that look like to you?
VII.1. One page taken from the Death Note, or even a fragment of the page, contains the full effects of the note.
Like the Shinigami eye deal, this rule is a test for the user, this time of their ingenuity. It's possible to cut out pages or fragments of pages and stash them elsewhere to be written on, which opens up limitless possibilities for the Death Note's user. One can be manipulated into writing names on a seemingly innocuous piece of paper, as is the case with Raye Penber. Coupled with two other rules—one which says that a name can be written with any material, another stating there are infinite pages—the owner of the Death Note is limited only by his own ambition and resourcefulness.
X.2. Whether the cause of the individual's death is either a suicide or accident, if the death leads to the death of more than the intended, the person will simply die of a heart attack. This is to ensure that other lives are not influenced.
Though it's somewhat far down a long, long list, this rule is one of the most important: the Death Note cannot make a victim kill. Write that the President of the United States orders a nuclear strike, and he will simply die of a heart attack. But why? In a notebook whose rules warn of unforeseen consequences, why would the Death Note care whether "other lives are not influenced?"
It might not be a question of minimizing impact per se, but rather of emphasizing the limitations of scale. Even a user of the Death Note has to share the planet with over seven billion other humans, and there's only so much an individual can do to change the world. Catastrophic events are rarely caused by one person acting alone—they are a result of many people making many decisions to act or not to act. Though the Death Note empowers one to kill, it remains as difficult to use it to cause a murder as it is to convince someone to commit one by speaking to them.
Difficult, but not impossible. The first live-action movie finds a way around this rule, but the circumstances of that exception would be prohibitively difficult to reproduce.
XVII.1. If the god of death decides to use the Death Note to kill the assassin of an individual he favors, the individual's life will be extended, but the god of death will die.
This rule directly causes Misa to enter the story. If the Shinigami Gelus hadn't taken pity on her and killed her assailant, his fellow death god Rem wouldn't have intervened in her life and given her the Death Note. Who can say where the series would have gone from there? But to step away from questions directly concerning the plot, this rule also underscores an unbreakable tenet of Shinigami law: one cannot extend life, only shorten it.
But that's a bit of an oversimplification. Another rule states that "the use of the Death Note in the human world sometimes affects other humans' lives or shortens their original life span, even though their names are not actually written in the Death Note itself." In other words, altering the balance of life and death is inevitable when one kills. Even humans know that.
What this rule seems to prevent is true altruism on the part of a Shinigami: unlike a human, a god of death cannot knowingly choose to allow someone to live longer. But that other rule's vagueness speaks volumes; it's impossible to know the full ramifications of a murder. If Shinigami are only able to use the Death Note for selfish purposes—on pain of death—what does that say about the humans who use it for a so-called greater good?
XXI.1. Those with the eye power of the god of death will have the eyesight of over 3.6 in the human measurement, regardless of their original eyesight.
3.6 is the Japanese equivalent of 72/20 eyesight—in other words, one can see 3.6 times the detail in any image as someone with "perfect" 20/20 vision. It's a perk of having the Shinigami eyes that's tangential to their function of discerning identities; even perfect human vision is a step below a god of death's ability to see names and lifespans. So what bearing does this have on the series? Exactly none, and that's part of what makes it great.
Story-wise, nothing would change if this rule didn't exist. The fact that it does is just one example of the careful consideration that went into crafting the world of Death Note. The rules I've outlined here are ten out of over a hundred in sixty-seven sections, which govern every possible situation and eventuality surrounding the Death Note and Shinigami/human relations. What if the Death Note is destroyed after a name is written in it? What if you use several pages to write a name? What if you misspell it? An essential part of building a world is making sure it has rules, especially when writing characters that bend and break them. Constraints drive creativity and conflict. Rule XXI.1 isn't just a throwaway joke—it's a symptom of a living, breathing story, one that is fully aware that the devil is in the details.