The Tortoise is a vote-counting protocol that is used in order to finalize block validity. For each block, the Tortoise constantly tallies all the votes both for and against it. Once a confirmation threshold is passed, it announces whether the block is valid or invalid. Ideally, the validity results of the Hare and the Tortoise should be in agreement with one another. However, the Tortoise guarantees that Smeshers will reach consensus, even if the Hare protocol fails (such a failure is extremely unlikely, but over a long-enough time period even very low probability events may eventually occur). Thus, even though the Tortoise is slower than the Hare, it ensures that consensus is reached on the entire history of the ledger.