Some lucky couples have a couple that has been their friends for decades. Usually, they live close to each other, visit often, maybe take vacations together and, often, their children are friends as well.

Both couples benefit from the relationship, but such longtime friendships aren’t easy to get.

Sociologists at the University of Maryland have conducted research on 426 people, published in their new book, Two Plus Two: Couples and Their Couple Friendships.

Co-authors Geoffrey Greif and Kathy Deal (married 36 years) say being close to another couple and watching how they handle ups and downs is a role model for how you can manage your own.

It enhances the relationship with your own partner. When you see your husband or wife with other people, it makes you appreciate them in a different way. You can see how charming or thoughtful your partner is or what a sparkling conversationalist he or she is, and it makes you proud.

And each has a chance to have a close, but not sexual, relationship with another man or woman they would otherwise not have.