Relationship
The discovery or establishment of common ground between individuals is a fundamental component for enduring interpersonal relationships. Loss of common ground, which may happen over time, may tend to end interpersonal relationships.
For each relationship type, essential skills are needed, and without these skills more advanced relationships are not possible. Systemic coaching advocates a hierarchy of relationships, from friendship to global order. Expertise in each relationship type (in this hierarchy) requires the skills of all previous relationship types. (For example partnership requires friendship and teamwork skills).
Interpersonal relationships through consanguinity and affinity can persist despite the absence of love, affection, or common ground. When these relationships are in prohibited degrees, sexual intimacy in them would be the taboo of incest.
Marriage and civil union are relationships reinforced and regularized by their legal sanction to be "respectable" building blocks of society. In the United States the de-criminalization of homosexual sexual relations in the landmark Supreme Court decision, Lawrence v. Texas (2003) facilitated the "mainstreaming" of gay long term relationships, and broached the possibility of the legalization of same-sex marriages in that country.
In intimate relationships there is often, but not always, an implicit or explicit agreement that the partners will not have sex with someone else - monogamy. The extent to which physical intimacy with other people is accepted may vary. For example, a husband may be more receptive to his wife being physically affectionate with her female friend if she has one than with her male friend (see also jealousy).
In friendship there is some transitivity: one may become a friend of an existing friend's friend. However, if two people have a sexual relationship with the same person, they may be competitors rather than friends. Accordingly, sexual behavior with the sexual partner of a friend may damage the friendship. See love triangle.
Sexual relations between two friends may alter that relationship by either "taking it to the next level" or severing it. Sexual partners may also be friends: the sexual relationship may either enhance or depreciate the friendship.
The rise of popular psychology has led to an explosion of concern about one's interpersonal relationships (often simply called: "relationships"). Intimate relationships receive particular attention in this context, but Sociology recognises many other interpersonal links of greater or less duration and/or significance.
Relationships are not necessarily healthy. Unhealthy examples include abusive relationships and codependence.
Sociologists recognize a hierarchy of forms of activity and interpersonal relations, which divides them into: behavior, action, social behavior, social action, social contact, social interaction and finally social relation.