Feature is used when you need to save your nested model with the parent model. Use-Case: Suppose you have a model say Person and it can be associated with N number of Contacts. Using nested form we can associate 1..n contacts to the person. Approach: For associating a contact to a person, we need to build the contact model with respect to the person model. For has_many relation, @contact = @person.contacts.build and if has_one relation @contact = @person.build_contact. Now we can use the @contact in the view, for this we use fields_for tag. And