Dwelling units are classified by occupancy and tenure into four groups: Owner occupied; tenant-occupied; vacant, for sale or rent; and vacant, not for sale or rent. Dwelling units are classified as occupied if occupied at the time of enumeration by households enumerated at their usual place of residence. The relatively small group classified as vacant, not for sale or rent" includes dwelling Units that were neither occupied nor available for sale or rent at the time of enumeration because they were being held for absent households, and dwelling units that were occupied temporarily by nonresident households.

A dwelling unit is defined as the living quarters occupied by, or intended for occupancy by, one household. A dwelling unit may be a detached house; a tenement, flat, or apartment in a larger building (apartment house, apartment hotel, or section of a hotel devoted entirely to apartment rather than transient use); or a room in a structure used primarily for business or other nonresidential purposes. It may be a tourist cabin, trailer, railroad car, boat, etc., if occupied by person having no other place of residence.

Three major race classifications are distinguished in this bulletin, namely, white, Negro, and other races. Persons of Mexican birth or ancestry who were not definitely Indian or of other nonwhite race were returned as white in 1940. In tables 3a and 6a statistics for the nonwhite (Negroes and other nonwhite races combined) are shown separately for tracts with 250 or more nonwhite persons. Occupation statistics for nonwhites are presented for Southern cities only. The white population is divided into two nativity groups, native and foreign born. A person born in the United States or in any of its territories or possessions is counted as native. The foreign-born white are presented by country of birth. The country of birth classification is based on political boundaries as of January 1, 1937, and therefore includes such countries as Austria; Czechoslovakia, and Poland.
The age classification is based on the age of the person at his last birthday before the date of the census, that is, the age of the person in completed years.