Partner System

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Partner systems are when systems are in a relationship. It can be romantic, platonic, or any other type of relationship.

Often, this means that many of the headmates in each system are in a relationship with members of the other system(s). Not all of them need to be, but this term does stand apart from two individual members within a system being in a relationship.

Related Terms
Spouse systems are a type of partner system that are married in shared reality. Specific terms for different kinds of relationships can replace "partner", like squish and plush.