Take the example of our spinner. We have seen that,....

“ Take the example of our spinner. We have seen that, to daily reproduce his labouring power, he must daily reproduce a value of three shillings, which he will do by working six hours daily. But this does not disable him from working ten or twelve or more hours a day. But by paying the daily or weekly value of the spinner's labouring power the capitalist has acquired the right of using that labouring power during the whole day or week. He will, therefore, make him work say, daily, twelve hours. Over and above the six hours required to replace his wages, or the value of his labouring power, he will, therefore, have to work six other hours, which I shall call hours of surplus labour, which surplus labour will realize itself in a surplus value and a surplus produce. If our spinner, for example, by his daily labour of six hours, added three shillings' value to the cotton, a value forming an exact equivalent to his wages, he will, in twelce hours, add six shillings' worth to the cotton, and produce a proportional surplus of yarn. As he has sold his labouring power to the capitalist, the whole value of produce created by him belongs to the capitalist, the owner pro tem. of his labouring power. By advancing three shillings, the capitalist will, therefore, realize a value of six shillings, because, advancing a value in which six hours of labour are crystallized. By repeating this same process daily, the capitalist will daily advance three shillings and daily pocket six shillings, one half of which will go to pay wages anew, and the other half of which will form surplus value, for which the capitalist pays no equivalent. It is this sort of exchange between capital and labour upon which capitalistic production, or the wages system, is founded, and which must constantly result in reproducing the working man as a working man, and the capitalist as a capitalist. ”

Karl Marx

From : Wage-Labour and Capital/Value