Logical Relationships between two activities in Project Schedule Management

The four relationships as explained in PMBOK® Guide 5th Edition are:

1. Finish-to-start (FS). A logical relationship in which a successor activity cannot start until a predecessor activity has finished. 

Examples: 

• The awards ceremony (successor) cannot start until the race (predecessor) has finished.
• Land must be purchased before road building can start. 

Once this task finishes, we can start the next one. In a Finish-to-Start relationship, the predecessor must finish before the successor can start. In fact, the predecessor’s finish date determines the Successor’s start date.

2. Finish-to-finish (FF). A logical relationship in which a successor activity cannot finish until a predecessor activity has finished.

Examples:

• Writing a document (predecessor) is required to finish before editing the document (successor) can finish or 
• Laying Asphalt must be completed before line painting can be completed.

We want these two tasks to finish at the same time. Once the predecessor task finishes, the successor task can finish.

3. Start-to-start (SS). A logical relationship in which a successor activity cannot start until a predecessor activity has started.

Examples:

• Level concrete (successor) cannot begin until pour foundation (predecessor) begins.
• Road excavating must start before asphalt can be laid. 

We want these two tasks to start at the same time. Once the predecessor task starts, we can start the successor task.

4. Start-to-finish (SF). A logical relationship in which a successor activity cannot finish until a predecessor activity has started.

Examples:

• The first security guard shift (successor) cannot finish until the second security guard shift (predecessor) starts or 
• Road excavating must start before line painting can be completed.

This task finishes when the next one starts, but not before then. Once the predecessor task starts, the successor task finishes.

References: PMBOK® Guide 5th Edition