So, for example 1 metre of Linen = 10 kilos of Sugar.
Here, 1 metre of Linen is the Relative Form of Value. Its Value is being expressed relative to a quantity of some other Use Value - here 100 kilos of Sugar, which is its equivalent.
This relation can only exist because both contain Value. Linen and Sugar are clearly two different things, which cannot be compared. However, the fact that both contain Value, means that this common thing Value can be compared. Because both linen and sugar contain different amounts of value i.e. both require different amounts of labour-time to produce, the only way in which an equal amount of Value can appear on both sides of the equation is if the Value of the 1 metre of linen, is equated with the quantity of sugar that contains this same amount of Value.
The Relative form of Value can remain constant, whilst its Equivalent Form changes. For example,
1 metre of linen = 100 kilos of feathers.
Even the Equivalent Form measured in the same Use Value can change. For example,
1 metre of linen = 12 kilos of sugar.
That can be for several reasons.
- The Value of linen could rise, because more labour-time is now required for its production.
- The Value of sugar could fall, because less labour-time is now required for its production.
- The Value of linen could rise, and the value of sugar fall.
- The Value of both linen and sugar might rise, but the Value of linen rise by more than that of sugar.
- The Value of both linen and sugar might fall, but the Value of linen fall by less than that of sugar.
For example, suppose the Value of 1 Metre of linen = 10 hours labour-time.
The Value of 100 kilos of sugar is also = 10 hours of labour-time. So,
1 metre linen = 100 kilos sugar. But, suppose a rise in productivity means both linen and sugar can be produced in half the previous time. Now, the Value of 1 metre of linen = 5 hours labour-time, and the Value of 100 kilos of sugar also = 5 hours labour-time. The Value of both linen and sugar has been halved. But, we still have:
1 metre linen = 100 kilos sugar.
The Value has been halved, but the Exchange Value of linen remains constant measured in its Equivalent Form, sugar.
For a fuller discussion see.
Capital I, Chapter 1 - Part 4