[...] the early history of industrial capitalism is tied up with attempts by capital to extend the time of the working day in an absolute sense, thus manipulating the ratio between necessary time and surplus time. [...]

However, as Marx realized, this absolute extension of the working day cannot go on indefinitely. Unions and collective bargaining limited the length of the working day, forcing capital to increase the intensity of labor. The concept of relative surplus value initially meant the cheapening of consumer goods that reproduce labor-power, so that the amount of necessary time would be decreased. [...]