As a comparison we might say, a Field is 100 metres long. But, a table is 2 metres long. Using, metre as the measure of length, both the field and the table have a length in metres that is unaffected by any other relation. Whatever happens to the table has no consequence for the length of the field and vice versa. But, once we establish a relation between these two things, then changes in the length of one do have a consequence for this relation.
So, we can say that:
1 Field = 50 tables i.e. both these two things have the same length of 100 metres.
1 Field is the "Relative Form of Length", whereas 50 tables is the "Equivalent Form of Length".
But, if a change in either or both of these two occurs, it clearly disturbs this relation. If tables are reduced in size to 1 metre, then the equation will be,
1 Field = 100 tables.
In other words, length measured by a constant metric (metres) is only affected by changes to the thing being measured. However, length when measured against some other standard is affected both by changes to itself, and changes to the thing against which it is being compared.
The same is true in relation to the Value Form. Value is measured in terms of Abstract Labour-time. The Value of something can only change if the labour-time required for its production changes. But, its Value relative to something else changes not only with changes to its own Value, but also changes in the Value of the thing against which it is being compared.
This Value Form analysis, Marx uses to demonstrate the way commodities develop as Use Values produced for the purpose of Exchange. Their Value, then takes the form of Exchange Value, which is represented in this Value Form. Marx demonstrates how logically and historically this Value Form evolves, so that ultimately a single commodity arises as a Universal Equivalent Form of Value, and this commodity is then transformed into Money. The historical and logical development is:
- Use Values are sporadically brought into relation one with another, as primitive tribal communities come into contact e.g. in wedding ceremonies. So the Value Form is like that above:
1 ox = 3 goats
1 camel = 5 sheep
and so on.
- The more these communities develop and come into more regular contact so that trade begins, the more each group has some item it regularly trades. At this stage, trade is still done by the community, rather than by individual traders. The Value Form here represents this transition, so the thing regularly traded assumes the role of Relative Form of Value.
1 ox = 3 goats
1 ox = 7 sheep
1 ox = 2 camels
and so on.
- As trade develops further so that a wider range of Use Values are frequently traded, its necessary to be able to compare the Values of each of these regularly traded goods easily against some standard. Consequently, one commodity, frequently the one that was most traded like cattle, or salt, is singled out so that it can act as the measure of all these other goods. So,
3 goats = 1 ox
7 sheep = 1 ox
2 camels = 1 ox.
So now, the thing most frequently traded assumes the position of Equivalent Form of Value. Its limitation in its present state can be seen from the above. If I want to know the (Exchange) Value of 1 goat, it is equivalent to 1/3 ox!
For further discussion on this see: Capital I, Chapter 3