Where should I store a foreign key?

Question

If I have a relationship between two tables (both tables have their own primary keys) what should guide my decision as to which table should store the foreign key? I understand

Answer 1

Which table is the child in the relationship?
Answer that, and you know which table needs the foreign key column, referencing the parent's [typically] primary key. That's for a one-to-many relationship...

A many-to-many would require you to add a third table, using the keys from both of the two tables as it's primary key.