The Two-Degree Blood-Relation Rule

The Two-Degree Blood-Relation Rule is the observed limit governing which relatives may be drawn into the disasters surrounding Yomiyama North Middle School Third-Year Class 3. Formulated by Tatsuji Chibiki from records he had kept over time, it states that the effect reaches students and their blood relatives within two degrees, making family connection a condition of exposure and establishing a boundary around the phenomenon’s otherwise chaotic violence.

Definition and Discovery

Tatsuji Chibiki presents the rule as an empirical conclusion rather than a proven law. As the self-identified observer of the Yomiyama phenomenon, he compares the facts accumulated across successive disasters and identifies a recurring familial pattern. The affected range includes parents, grandparents, and siblings related by blood. Aunts, uncles, and cousins therefore fall outside the stated two-degree limit.

The rule also excludes relatives connected only through marriage or remarriage. Chibiki reports that no stepparent or stepsibling without a blood tie has ever died within the pattern he recorded, leading him to regard such relatives as outside the range. The distinction is consequently not simply one of household proximity: biological relation matters, while shared family life alone does not.

Relation to the Wider Range of the Phenomenon

The familial boundary is one part of a broader range structure. Chibiki explains that the disasters are centered on Yomiyama and the school, and that their effect weakens with increasing distance from the town. Geographic distance and blood relationship thus operate together: the phenomenon is not presented as an unlimited force that can reach anyone anywhere, but as something concentrated around a particular community and its immediate biological families.

This structure also clarifies why people apparently outside the class can remain comparatively safe. Chibiki tells Koichi Sakakibara and Mei Misaki that someone with no direct link to Third-Year Class 3 can interact with its members without being affected in the same way. His position as an observer and former teacher allows him to gather information while remaining outside the class’s direct chain of danger. The rule therefore distinguishes between participation in the class and mere contact with it.

Consequences for Records and Recognition

The rule operates within a larger system in which the extra person, also called the casualty, can hide inside the class while records and memories are altered. Chibiki confirms that details which should appear inconsistent are made to seem ordinary, preventing the class from recognizing the casualty’s true identity. The familial range consequently belongs to a world where even apparently reliable social records cannot be assumed to remain stable.

The class’s countermeasures likewise demonstrate the pressure exerted by this system. After the attempt to treat both students as not there failed to produce any benefit, the class gradually stopped ignoring Mei and Koichi. That change did not eliminate the disasters; it merely showed that the social tactic had ceased to seem useful. The blood-relation rule remains a more durable description of the phenomenon’s reach than the class’s shifting decisions about whom to acknowledge.

The Numata Case

The deaths of Mieko Numata and Kensaku Numata provide a concrete application of the rule. Tatsuji Chibiki investigated further and discovered that the Numatas were the grandparents of Ikuo Takabayashi on his mother’s side. Since grandparents fall within the two-degree blood range, their deaths could be counted among the disasters connected with the class.

This discovery changes the apparent number of deaths for August. Mei Misaki states that there were seven deaths for the month, and Koichi asks whether the Numatas are being included. Their inclusion is not arbitrary: their relationship to Takabayashi places them inside the defined familial boundary, even though they were not members of the class themselves. The case turns a seemingly isolated pair of deaths into evidence supporting Chibiki’s rule.

Significance

The Two-Degree Blood-Relation Rule gives the disasters a grim form of order. It narrows the threat to a network defined by class membership, blood ties, and proximity to Yomiyama, while leaving non-blood relatives and people without a direct class connection comparatively outside it. At the same time, the rule does not make the phenomenon controllable. Knowing who may be endangered does not reveal when the disasters will strike, and altered memories can conceal the very relationships needed to interpret the pattern.

Its importance is therefore both practical and interpretive. The rule supplies one of the few stable principles in a narrative built on corrupted records, uncertain identity, and recurring death. Yet its precision intensifies rather than dispels the horror: a grandparent’s connection to one student can extend the class’s catastrophe into a family that otherwise appears remote, making ordinary kinship itself part of the mechanism of danger.