 According to Kripke, a proper understanding of rigidity also reveals that some contingent statements are a priori. Kripke considers the standard meter stick, which has been used to define the length of one meter. Someone decreed that the expression ‘one meter’ is to be used for the unit of length of that stick, perhaps at a certain time in the stick’s existence. The stick, S, might not have been the length that it was at the time ‘one meter’ was coined: had it been heated or cooled it might have been longer or shorter. As things are, it was not heated or cooled. So the sentence ‘One meter = the length of S (at time t0)’ is true in the actual world but it is false in some possible worlds: it is contingently true. Yet S has been used to define ‘one meter’, in the sense that it fixes the reference (see above), so the definers of ‘one meter’ know a priori the truth of the sentence ‘The length one meter is instantiated by S (at time t0)’. The reason that the sentence is contingently true is that ‘one meter’ is a rigid designator for the length one meter. ‘The length of S (at time t0)’ is, on the other hand, a non-rigid designator for one meter; in this world it designates one meter but in other worlds, those in which S is heated or cooled, ‘the length of S (at time t0)’ designates other lengths.