Full Idea
In 'one meter is the length of stick S at t', one designator (one meter) is rigid and the other (length of S at t) is not. 'S is one meter long at t' is contingent, as it could have a different length. In this sense, there are contingent a priori truths.
Gist of Idea
The meter is defined necessarily, but the stick being one meter long is contingent a priori
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 1)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.56
A Reaction
[very compressed] Not convincing. He is proposing that a truth is knowable a priori, though knowledge of it is utterly dependent on a ceremony having taken place. It would not be true if that event hadn't taken place, so how can be it be known a priori?