In a project network

Project network

A project network is a graph depicting the sequence in which a project's terminal elements are to be completed by showing terminal elements and their dependencies....

, a dependency is a link amongst a project

Project

A project in business and science is typically defined as a collaborative enterprise, frequently involving research or design, that is carefully planned to achieve a particular aim. Projects can be further defined as temporary rather than permanent social systems that are constituted by teams...

's terminal elements.

There are four kinds of dependencies with respect to ordering terminal elements (in order of decreasing frequency of use):

1. Finish to start (FS)
   • A FS B = B can't start before A is finished
   • 
   • (Foundations dug) FS (Concrete poured)
2. Finish to finish (FF)
   • A FF B = B can't finish before A is finished
   • 
   • (Last chapter written) FF (Entire book written)
3. Start to start (SS).
   • A SS B = B can't start before A starts
   • 
   • (Project work started) SS (Project management activities started)
4. Start to finish (SF)
   • A SF B = B can't finish before A starts
   • 
   • (New shift started) SF (Previous shift finished)

Finish-to-start is considered a "natural dependency" whereas all the others are constraints imposed by the scheduler to reflect resource constraints or preferential dependencies.