Paul and Jane are married and are each others' primary relationship. Jane has a secondary relationship with Karl, who would be Paul's tertiary relationship. These people are practicing polyamority.
PolyAmorists tend to focus more on the interpersonal and romantic connections with their partners, thus differentiating their practice from Swingers. Sexual activity does occur, just as it does in monogamous marriages.
Sue was in a polyamorus relationship where she had love making trysts with both Robert and Charles during the week and no one was angry, not even her husband William.