Those involved are faithful, and can not relate lovingly or sexually to people outside the relationship.
Everyone is free to relate as they please, with people outside the relationship.
It happens when people have relationships with each other and everyone interacts.
When three people date, but two have a stronger relationship. Usually occurs when a couple dates a third person.
It happens when everyone gets directly involved. They can involve men and women and usually all are bisexual.
It consists of four members and there may be a relationship between two or more people in the group.