Habermann's method of deadlock preventative is discussed, where deadlock is defined as a system state from which resource allocations to certain processes is not possible. It is shown that the scheduler may introduce deadlocks which Habermann's method does not prevent. Effective deadlock is defined as the situation where certain processes do not receive their resource requests. It is shown that deadlock prevention does not imply effective deadlock prevention. A method of effective deadlock prevention is given. Key Words and Phrases: multiprogramming, time-sharing, scheduling, resource allocation, deadlock, interlock, deadly embrace, knotting.