Condition variables are synchronization objects that allow threads to wait for certain events (conditions) to occur. Condition variables are slightly more complex than mutexes, and the correct use of condition variables requires the thread to co-operatively use a specific protocol in order to ensure safe and consistent serialization. The protocol for using condition variables includes a mutex, a boolean predicate (true/false expression) and the condition variable itself. The threads that are co-operating using condition variables can wait for a condition to occur, or can wake up other threads that are waiting for a condition.