A Death Note appears as a basic notebook with different designs based on the owner. Most are plain black and don't have writing on the cover, such as that used by the Shinigami Rem. However, some shinigami may add their own personalization to the Death Note, whether it's something on the front (usually their name or the words "death note"), something on the inside cover (such as the rules on the inside of the notebook Light Yagami got from Ryuk), or even another colour, such as red or white, although these aren't seen in the series. The Death Note can take the form of other similar objects, such as a scroll, depending on the era humanity is in at the time.
The purpose of the Death Note is to take human life, thereby increasing the life of the Shinigami who uses it. There are many rules as to how the Death Note can be used, the most pivotal of which is that "The human whose name is written in this note shall die." Shinigami normally use Death Notes to kill humans from the Shinigami Realm, thereby adding each human's remaining years of life to their own.
Rogue Shinigami are Shinigami who drop extra Death Notes from the Shinigami Realm down to the human world in order to allow humans the use of their spare killer notebooks. It is unknown how many notebooks exist in the Shinigami Realm, as well as how Death Notes are created. However, only six Death Notes may exist in the human world at any given time. A Death Note can be used by a human, although their lifespans do not increase by doing so.
There are many rules regarding the use of the Death Note, all of which can be found here. Some of the more important rules have been added below.
- The human whose name is written in this note shall die.
- The human who becomes the owner of the Death Note can, in exchange of half his/her remaining life, get the eyes of the Shinigami which will enable him/her to see a human’s name and remaining life span when looking through them.
- 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.
- 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.
- If a Death Note owner accidently misspells a person's name four times, that person will be free from being killed by the Death Note. However, if the Death Note owner intentionally misspells the name four times, the owner will die.