ことになっている

The grammar structure ことになっている expresses established rules, customs, expectations, or scheduled arrangements. It describes situations where something has been decided – typically not by the speaker personally, but by an external authority, social convention, or prior arrangement. It conveys that "it is the rule/expectation that…" or "it has been arranged that…"

Rules, regulations, and customs

ことになっている is used to describe rules, regulations, or socially accepted customs. The emphasis is on an established standard that people are expected to follow.

With the negative form ない + ことになっている, it expresses that something is not supposed to be done.

Scheduled arrangements

ことになっている can also describe something that has been scheduled or arranged, often by someone other than the speaker or through a group decision.

Compared to ことにしている

While ことになっている implies that a rule or decision was established externally (by society, an organization, or circumstances), ことにしている expresses a personal decision or habit that the speaker has chosen for themselves.

Compared to ことになった

ことになった focuses on the moment a decision was made ("it has been decided that…"), while ことになっている describes the ongoing state of that decision being in effect ("it is the established rule/arrangement that…").