Approximately 134 million households participated in the 2010 census by mail. Enumerators conducted door-to-door interviews only for households that could not be reached by the U.S. Postal Service or did not return their mailed questionnaire. The instructions provided here were used by enumerators who visited households that did not return their mailed questionnaire during the 2010 census' "Nonresponse Follow-Up" (NRFU) operation.