Socially Ambivalent Labour Time V: TSV part 3, chapter 20

Marx's discussion of socially necessary labour time in Chapter 20 of Theories of Surplus Value is notable for the fact that it comes immediately before Chapter 21, where he doesn't mention socially necessary labour time (but the concept lingers just below the surface in the latter chapter). He talked about how piece-work is actually a kind of time work in which the piece rate is set according to expectations of how many pieces can be made in a designated period of time.

Once again Marx reiterated the notion of value being measured in terms of abstract labour: simple, uniform, average labour. "That the quantity of labour embodied in a commodity is the quantity socially necessary for its production -- the labour-time being thus necessary labour-time -- is a definition which concerns only the magnitude of value.