In the first part of this paper it is shown that unit names, whether simple or complex, whether of fundamental, associative or derivative measurement, may always be regarded as the names of scales. In the second it is shown that dimension names, whether simple, like "[M]", "[L]" and "[T]", or complex dimensional formulae, may always be regarded as the names of classes of similar scales. Thus, a new foundation for the theory of dimensional analysis is provided, and in the light (...) of this, its nature and scope are examined. Dimensional analysis is shown to depend upon certain conventions for expressing numerical laws. (shrink)