Exploitation is a matter of surplus labour—the amount of labour performed beyond what is received in goods. Exploitation has been a socioeconomic feature of every class society and is one of the principal features distinguishing the social classes. The power of one social class to control the means of production enables its exploitation of other classes. Under capitalism, the labour theory of value is the operative concern, whereby the value of a commodity equals the socially necessary labour time required to produce it. Under such condition, surplus value—the difference between the value produced and the value received by a labourer—is synonymous with the term "surplus labour" and capitalist exploitation is thus realised as deriving surplus value from the worker.