The elementary form of value of a commodity is contained in the equation, expressing its value relation to another commodity of a different kind, or in its exchange relation to the same. The value of commodity A, is qualitatively expressed, by the fact that commodity B is directly exchangeable with it. Its value is quantitatively expressed by the fact, that a definite quantity of B is exchangeable with a definite quantity of A. In other words, the value of a commodity obtains independent and definite expression, by taking the form of exchange value.
Karl Marx, Capital Vol 1















