In converting a one-to-one binary relationship in an E-R diagram into two relational tables, the rule is that the unique identifier of the entity on the "many side" of the one-to-many relationship is placed as a foreign key in the table representing the entity on the "one side."

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The question refers to a one-to-one relationship, not a one-to-many relationship. In a one-to-one relationship, either entity's identifier can be used as a foreign key in the other entity's table. It is not necessary to place the foreign key in one entity's table exclusively.

In Converting a One-To-One Binary Relationship in an E-R Diagram