We study the issue of data consistency in highly-available distributed systems. Specifically, we consider a distributed system that replicates its data at multiple sites, which is prone to partitions, and which is expected to be highly available. In such a setting, strong consistency, where all replicas of the system apply synchronously every operation, is not possible to implement. However, many weaker consistency criteria that allow a greater number of behaviors than strong consistency, are implementable in distributed systems. We focus on determining the strongest consistency criterion that can be implemented in a distributed system that tolerates partitions. We show that no criterion stronger than Monotonic Prefix Consistency (MPC) can be implemented. MPC is the consistency criterion underlying blockchains.