Byzantine fault tolerant state machine replication (BFT-SMR) is a foundation for implementations of highly reliable services. Existing algorithms for BFT-SMR require at least 3f +1 active replicas to tolerate f faulty replicas. We show that BFT-SMR can be achieved with fewer than 3f +1 active replicas, as long as standby spare replicas are available, such that the number of active