See Relationship Type for the attributes. None are required, but you will normally send at least forward and reverse. For a relationship that reads the same both ways, such as "is the spouse of", use the same text for both.

{
  "forward": "is the parent company of",
  "reverse": "is the subsidiary of",
  "forward_title": "Parent Company",
  "reverse_title": "Subsidiary",
  "for_contact": false,
  "for_company": true
}

New relationship types are active and, unless you say otherwise, enabled for both contacts and companies. A successful request returns 201 Created with the new type's reference, e.g. {"location": "/v1/relationship_types/1000"}.