In Chapter 2 we saw that early Marxist theorists formulated an idea about working-class unity and solidarity, but that they rarely applied the concept of solidarity in their texts. Chapter 4 concluded that this was the case in the party programmes in the first decades of the labour movement parties as well. Other terms such as worker unity, (proletarian) internationalism and fraternity or brotherhood were more frequently applied. Such terms were sometimes functional equivalents to solidarity, e.g. fraternity and brotherhood. At other times they referred to different specific aspects of solidarity such as unity in the struggle for tariffs and better working conditions, or to sympathy and material support for workers in other nations. Both solidarity and other more or less functionally equivalent terms were based on a notion of class interest, were restricted to the working class and implied a strong degree of collective orientation where individual autonomy had to be subordinated to the interest of the collective. Later, solidarity replaced the other equivalent terms and was gradually redefined and transformed into a new concept with a different foundation, goal, inclusiveness and degree of collective orientation. The transformation of the Marxist to the modern social democratic idea of solidarity entailed a change of all these four aspects of solidarity and the relationship between them.