Capital, Volume One: The Process of Production of Capital by Karl Marx

Wages by the piece are nothing else than a converted form of wages by time, just as wages by time are a converted form of the value or price of labour-power.

In piece wages it seems at first sight as if the use-value bought from the labourer was, not the function of his labour-power, living labour, but labour already realized in the product, and as if the price of this labour was determined, not as with time-wages, by the fraction daily value of labour-power the working day of a given number of hours but by the capacity for work of the producer. [Footnote 1]