Introduction. Many automatic computers employ magnetic drums or ultrasonic delay lines as their main store. A part of the time taken by a computation on such a machine is spent in waiting for words to become available from the store. A number of methods and devices have been used or suggested to reduce this waste of time. This paper attempts to evaluate these methods by determining on the one hand the amount of waiting time eliminated and on the other hand the design or programming complications introduced. No quantitative method for determining the amount of eliminated waiting time was known. Available estimates were based on the feeling of the programmer and varied accordingly. It was therefore necessary in the first place to develop a process by which the amount of waiting time eliminated by any given method could be determined. This process is set out while being applied to the methods discussed. The methods themselves are not new, except that closer analysis has shown that the method adopted with the ACE pilot model and known as ‘optimum programming’ is in fact not a single method but a combination of several methods with different features. Also, it appeared that, contrary to an opinion expressed elsewhere (2), there is a simple way of staggering locations in the store even with words of two lengths.