" Commodities are the embodiment of socially necessary labour time. But
labour time as such is not expressed directly, as it is in the society
envisaged by Rodbertus, in which the central authority establishes the
unit of labour time which it will accept as valid for each commodity.
Labour time is expressed only in the exchange commensurability of two
articles. Thus the value of an article, i.e., its average time of
production, is not expressed directly as eight, ten or twelve hours, but
as a specific quantity of another article. In other words, a natural
object with all its material attributes expresses the equivalent value
of another thing. For example, in the equation, one coat equals twenty
metres of linen, the twenty metres of linen are the equivalent of one
coat simply because both are embodiments of socially necessary labour
time. It is in this sense that all commodities are commensurable. "

― , Finance Capital: A study in the latest phase of capitalist development