A general approach to scheduling in which the item-processing times ar
e given as intervals is formulated using the example of the well-known
, problem of two machines. The set of optimal schedules of the interva
l processing system is shown to be the intersection of the sets of opt
imal schedules of its lower and upper boundary systems. From this fact
the existence conditions of optimal schedules of an interval system a
re established as are the decision rules for their determination.