Connecting the activities and milestones with sensible logic is the bedrock of any schedule model. The method of connection is defined as a relationship. Every activity and milestone except the first and last must be connected to at least one predecessor and one successor. With the exception of the start milestone something must occur prior to an activity starting and that activity must be totally or partially completed to allow another activity to start.
Ensuring compliance with this practice will prevent the schedule from containing open ends, where activities or milestones are missing predecessors or successors. Some schedulers may be tempted to use lags to represent a period of time when work is actually occurring such as review of a document before the next phase proceeds. It is recommended that these types of work be shown as activities in the schedule model instead of using a lag. Then such activities are included and they could be coded to show these are activities for which another party, for example, the client is responsible. This practice allows for better control of the project and makes it easier to change the review duration.
It is also possible to assign constraints to activities and milestones which required the activity or milestone to start or finish at specific points in time. The Project Manager is strongly encouraged to study the various types of constraints that might be used and understand the effect and nuance their use has upon the schedule. The generally accepted practice is that constraints and lags should not be used to replace schedule logic.