Mésalliance

A mésalliance is a marriage to an unsuitable partner. Typically used to define a union with a socially inferior partner, like morganatic marriage by a member of royal family, this Gallicism is also used metaphorically, especially in the misalliance variant, to describe a generally unworkable association, for example, the ill-fated alliance of German nobility with Hitler.
Researchers also use terms hypergamy and hypogamy to describe marriages involving partners from different social classes or status. Both terms were invented on the Indian subcontinent in the 19th century while translating classical Hindu law books, which used the Sanskrit terms anuloma and pratiloma. The hypergamy and hypogamy can therefore be considered as special cases of mésalliance.
Mesalliances break the patterns of endogamy. The definitions of the "proper marriage" and consequences of ignoring them varied from one culture to another.

Nobility

For nobles, mésalliance denoted a scandal of a marriage between an aristocrat and a person of lower standing that might involve a loss of rank or inheritance. Although in the aristocratic circles this looked like a disaster, the noble misalliances were not infrequent, driven by either romantic or financial considerations.

France

The word mésalliance, originally of a French origin, had been used in English since the late 18th century. The French society of the 17th-18th centuries was built not of individuals, but of the families. These patriarchal families were living, for many generations, in a family house, a heritage from the ancestors that had to be preserved for the future generations at any cost. Every family member had their own hereditary social position and counted for nothing without the family. A mesalliance represented a direct threat to this system. Marrying for the status and wealth was the norm, so when the young Louis XIV was infatuated with Marie Mancini, he was forced to marry the Maria Theresa of Spain instead, with "father of all the fathers" thus adding Franche-Comté and part of the Flanders to the possessions of his "large family". But on the question of mésalliance the French Third Estate was "just as susceptible" as nobility.
During the 18th century, the nobility gradually lost all fear of mesalliances based on descent, and the highest nobility was marrying for money. Nicolas Chamfort remarked that "almost all the women..., either in Versailles or Paris..., are only bourgeoises de qualité".

In art

Mesalliances had attracted attention of writers and artists since antiquity. The story of Apollonius of Tyre, an ancient Greek novel, describes Apollonius, a young king disguised as a fisherman, successfully wooing a princess. The tradition carried over through Mediaeval period to modern times with the stories of George Dandin, chevalier des Grieux, and the play Misalliance by Bernard Shaw.
Painters were attracted to the subject since at least the late 15th century, typically playing on a contrast between a young wife and her old husband, sometimes reversing the ages. Notable works were created by Quentin Matsys and Lucas Cranach the Elder.