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A project network is a graph (weighted directed graph) depicting the sequence in which a project's terminal elements are to be completed by showing terminal elements and their dependencies. It is always drawn from left to right to reflect project chronology.
The most popular form of project network is activity on node (AON) (as shown above), the other one is activity on arrow (AOA).
The condition for a valid project network is that it doesn't contain any circular references.
Project dependencies can also be depicted by a predecessor table. Although such a form is very inconvenient for human analysis, project management software often offers such a view for data entry.
An alternative way of showing and analyzing the sequence of project work is the design structure matrix or dependency structure matrix.