A married couple with or without own children, or one parent with one or more own children (parent-child group), living in a housing unit and related to the householder, but excluding the householder (for example, a young married couple sharing the home of the husband's or wife's parents). Since subfamily members are counted as part of the householder's family, the number of subfamilies is not included in the count of families per se. Subfamilies are defined during processing of sample data. In selected tabulations, subfamilies are further classified by type: married-couple subfamilies, with or without own children; father-child subfamilies; and mother-child subfamilies.

(In certain Census Bureau surveys (e.g. CPS) before 1980, families as defined here are referred to as "primary families." The term "secondary family" refers to a resident family unrelated to the householder, such as a roomer and his or her spouse. Tabulations of families from such surveys include secondary families.)