If social units are to be classified it must be by reference to some distinctive characteristic or characteristics that they share. Administrative classifications are usually based on the characteristics identified in the everyday language that reflects practical knowledge. Classifications that will assist the growth of social scientific knowledge have to be based on the identification of theoretically relevant characteristics. Classification precedes the naming of categories. Experimental research into the relative strength of civic and ethnic preferences could uncover the variables that underlie popular notions of nation, race and ethnic group.