About Task Dependency Relationships and Lag

In Project Task Gantt you can create a task dependency relationshipClosed between two tasks using dependency types. You can also specify a lagClosed time on a task dependency relationship.

Note: Note
When you create a task dependency relationship in Project Task Gantt, the default lagClosed time of 0 is applied. When you change the dates of a dependent task, for example by dragging the task bar on the Schedule pane, the preceding task's dates are not updated in accordance with dependency rules.
To re-apply dependency rules:
  • Right-click a task and select Apply Dependency Rules.
  • Click Resolve Dependencies .
When you save changes dependency rules are not re-applied.
You cannot create a dependency relationship between a parent task and any other task.

Task Dependency Relationship Types

There are several types of dependency relationships that you can apply between two tasks. The first task in the relationship is known as the predecessor and the second called the successor. Dependency types and descriptions are provided here:

Dependency Type Relationship Description

Default Value
Finish to Start (FS)Closed The successor project task cannot start until the predecessor project task is complete. Yes
Start to Finish (SF)Closed The successor project task cannot finish before the predecessor project task starts. No
Start to Start (SS)Closed The successor project task cannot start until the predecessor project task starts. No
Finish to Finish (FF)Closed The successor project task cannot finish until the predecessor project task is complete. No

When a dependency is created between two tasks, the successor task's details are recalculated according to the dependency type you apply. The recalculation is also influenced by which duration unit each task is set to. The predecessor task does not move unless you manually change its details. When there are more than two tasks in a dependency, the predecessor task's details are recalculated appropriately when it is also a successor to another predecessor task.

About Lag Time on Task Dependency Relationships

Lag can be applied when there is a dependency relationship between two project tasks. The lag is the time gap added or subtracted from the end of a predecessor task before the successor task can start. The default lag is 0. Lag time can be a positive or negative value. For more information, see the description for the Lag field.

Formulas recalculate project task start or end dates and times accordingly in dependencies when you update the dependency’s lag time.

Dependency Type

Recalculation

Lag Formula Used

Finish to Start

Start of the successor task. Finish of the predecessor task plus or minus any applied lag time.
Start to Finish Finish of the successor task. Start of the predecessor task plus or minus any applied lag time.
Start to Start Start of the successor task. Start of the predecessor task plus or minus any applied lag time.
Finish to Finish Finish of the successor task. Finish of the predecessor task plus or minus any applied lag time.
Predecessor Successor Successor Duration Unit Lag Time Applied Recalculated Successor With Lag
Mon 01/01 9am - 5pm Mon 01/01 9am - 5pm Days 2d Tue 03/01 9am - 5pm
Mon 01/01 9am - 5pm Mon 01/01 9am - 5pm Days -2d Wed 28/12 9am - 5pm
Mon 01/01 9am - 5pm Mon 01/01 9am - 5pm Days 2h Mon 01/01 9am - 5pm
Mon 01/01 9am - 5pm Mon 01/01 9am - 5pm Days -2h Mon 01/01 9am - 5pm
Mon 01/01 11am - 3pm Mon 01/01 10am - 4pm Hours 2d Wed 03/01 5am - 11am
Mon 01/01 11am - 3pm Mon 01/01 10am - 4pm Hours -2d Sat 30/12 5am - 11am
Mon 01/01 11am - 3pm Mon 01/01 10am - 4pm Hours 2h Mon 01/01 7am - 1pm
Mon 01/01 11am - 3pm Mon 01/01 10am - 4pm Hours -2h Mon 01/01 3am - 9am