Plaçage was a recognized extralegal system in French slave colonies of North America (including the Caribbean) by which ethnic European men entered into civil unions with non-Europeans of African, Native American and mixed-race descent. The term comes from the French placer meaning "to place with". The women were not legally recognized as wives but were known as placées; their relationships were recognized among the free people of color as mariages de la main gauche or left-handed marriages. They became institutionalized with contracts or negotiations that settled property on the woman and her children and, in some cases, gave them freedom if they were enslaved. The system flourished throughout the French period, reaching its zenith during the latter, between 1769 and 1803.

The system may have been most widely practiced in New Orleans, where planter society had created enough wealth to support the system. It also took place in the Latin-influenced cities of Natchez and Biloxi, Mississippi; Mobile, Alabama; St. Augustine and Pensacola, Florida; as well as Saint-Domingue (now the Republic of Haiti). Plaçage became associated with New Orleans as part of its cosmopolitan society.

Emily Clark has challenged the popular notion of plaçage as being a systemic practice based on contractual marriages, and proposes that the practice largely consisted of a broad range of relationships between free women of color and white men that originated in various ways, which often lasted for a lifetime. Genetic evidence strongly supports the historical record of mass intermarriages between white men and women of color in New Orleans.

The plaçage system developed from the predominance of men among early colonial populations, who took women as consorts from Native Americans, free women of color and enslaved Africans. In this period there was a shortage of European women, as the colonies were dominated in the early day by male explorers and colonists.

Given the harsh conditions in the colonies, persuading women to follow the men was not easy. France sent women convicted along with their debtor husbands and in 1719 deported 209 women felons "who were of a character to be sent to the French settlement in Louisiana."

The placage system first developed in Saint-Domingue. France sent women from the poor houses to the West Indies, but they had the reputation of also being former prostitutes from La Salpêtrière, and in 1713 and again in 1743, the authorities in Saint-Domingue complained that Paris sent the settlers unsuitable former prostitutes as wives. The custom of sending brides from France was therefore discontinued in the French West Indies in the mid-18th century, which benefitted the development of a placage system there. A new colonisation policy was adopted in which male colonists from France merely came to the colony to make their fortune and returned to France after a few years during which they did not marry but lived with a free woman of colour.

France also relocated young women orphans known as King's Daughters (French: filles du roi) to their colonies for marriage to both Canada and Louisiana. France recruited willing farm- and city-dwelling women, known as casket or casquette girls, because they brought all their possessions to the colonies in a small trunk or casket.

Historian Joan Martin maintains that there is little documentation that "casket girls", considered among the ancestors of white French Creoles, were brought to Louisiana. The Ursulines, an order of nuns, chaperoned the casket girls until they married. Martin writes that some Creole families who today identify as white had ancestors during the colonial period who were African or multiracial, whose descendants married whites over generations.