|Abstract:||In this paper, I discuss Finnish case alternation adpositions from a theoretical and corpus perspective. First, I argue that postpositional PP constructions with genitive case denote the standard spatial meaning of which an extension is marked with partitive case. Also, I show how word order interacts with case assignment. Both findings are formalized in a bidirectional Optimality Theoretic framework. Second, I show that case alternating behavior does not occur unrestrictedly in newspaper corpora. Adpositions in principle tend to assign the same case to the same object over and over again, and only a small subgroup of highly frequent nouns is assigned both genitive and partitive case by the same adposition(s). This suggests that (adpositional case) alternation is only allowed for highly frequent adpositional objects. Frequency is a necessary condition for variation (and therefore, for bidirectional optimization).