 Conventionalism with respect to measurement reached its most sophisticated expression in logical positivism. Logical positivists like Hans Reichenbach and Rudolf Carnap proposed “coordinative definitions” or “correspondence rules” as the semantic link between theoretical and observational terms. These a priori, definition-like statements were intended to regulate the use of theoretical terms by connecting them with empirical procedures (Reichenbach 1927: 14–19; Carnap 1966: Ch. 24). An example of a coordinative definition is the statement: “a measuring rod retains its length when transported”. According to Reichenbach, this statement cannot be empirically verified, because a universal and experimentally undetectable force could exist that equally distorts every object’s length when it is transported. In accordance with verificationism, statements that are unverifiable are neither true nor false. Instead, Reichenbach took this statement to expresses an arbitrary rule for regulating the use of the concept of equality of length, namely, for determining whether particular instances of length are equal (Reichenbach 1927: 16). At the same time, coordinative definitions were not seen as replacements, but rather as necessary additions, to the familiar sort of theoretical definitions of concepts in terms of other concepts (1927: 14). Under the conventionalist viewpoint, then, the specification of measurement operations did not exhaust the meaning of concepts such as length or length-equality, thereby avoiding many of the problems associated with  operationalism.[13]