The well-known coordinated snapshot algorithm of mutable checkpointing [7,8,9] is studied. We equip it with a concise formal model and analyze its operational behavior via an invariant characterizing the snapshot computation. By this we obtain a clear understanding of the intermediate behavior and a correctness proof of the final snapshot based on a strong notion of consistency (reachability within the partial order representing the underlying computation). The formal model further enables a comparison with the blocking queue algorithm [13] introduced for the same scenario and with the same objective.From a broader perspective, we advocate the use of formal semantics to formulate and prove correctness of distributed algorithms.