Through a study combining an analysis of the representation and variable significance of the expression “middle classes” in light of the principal social evolution that has occurred since the last third of the nineteenth century, this paper defends the following two theses: 1) The plural form was conveniently suited to a country where the middle classes held greater weight than elsewhere due to the Third Republic’s parliamentary democracy and unique balance of the powers.Because of its ambiguity, this social denomination facilitated consensus and incorporated several of the basic myths of the “republican compromise”: individualism, faith in social mobility and fluidity, attachment to small property (industrial, rural or commercial) and hostility towards the privileged. 2) The French social dynamic precociously accentuates (at least in the “mythified” representations) aspiration towards equality (especially equality of opportunity), which is manifested through a system of upward mobility in recognizable channels. During periods of uncertainty for this system of social dynamics, the conception of this group becomes more concrete and precise. This was the case in the interwar period or at the end of the fifties when France was confronted to a quick social change. Recently globalization called the accomplishments of the new middle classes during the “Thirty Glorious” years into question. On the other hand, during periods of prosperity and widening social mobility (in our times during the 1960s and 1970s),we return to a vague and ambiguous vision that encompasses the plural term. Three distinct periods of time may be defined: 1) The beginning of the Third Republic, when the modern sense of the plural form is defined; 2) The period of growing corporatism (c.1900-c.1970), when, throughout the debates regarding the role and existence of the middle classes, the ambiguities inherent in the plural form result in the mild management of political crises; 3) The period since the 1970s, when the change of economic climate, the decisive push for access to secondary and higher education,and the increasing number of women in senior job positions once again creates the sensitive social and political questions regarding the middle classes.The current public debates betray a sense of confusion and reveal a lasting incapacity to produce a new common project that will be neither addicted to the past nor limited to a particular segment of society.