Adding constraints to a project

Sometimes you might need to show that a task cannot start before a certain date or must be completed by a particular date. For example, you might not be able to start work on a task until certain permissions have been granted, or you might need to complete a task no later than a specific date in order to meet an interim deadline. You can show these external influences on the bar chart by using start flags and finish flags. These flags let you constrain a task to start or finish on, before, or after specific dates. Rescheduling a project positions tasks so that they conform to their constraints, even if this means that tasks overlap and links go backwards.

Types of constraint

Constraint Appearance Description
Start On or After A green flag appearing to the right of a bliue dot Constrains a task to start on or after a specific date. The task can be moved to a later date but cannot be moved to an earlier date when you reschedule the project. Successor tasks may be moved as a result of the task moving to a later date.
Start On A green flag appearing directly above a bliue dot Constrains a task to start on a specific date. The task is not moved when you reschedule the project, even if the internal logic of the plan dictates that the task could be moved earlier or later. Predecessor or successor tasks may be moved to allow the task to remain on its start date.
Finish On or Before A red flag appearing to the left of a bliue dot Constrains a task to finish by a specific date. The task can be moved to an earlier date but cannot be moved to a later date when you reschedule the project. Predecessor tasks may be moved to allow the task to remain on its finish date.
Deadline A blue flag appearing directly above a bliue dot If you apply a Deadline constraint to a task, the task is able to slip beyond the flag date. Deadline flags, which point in the opposite direction to other finish flags, appear as blue unless the task slips beyond them, in which case they appear shaded with diagonal stripes.
Finish On A red flag appearing directly above a bliue dot Constrains a task to finish on a specific date. The task is not moved when you reschedule the project, even if the internal logic of the plan dictates that the task could be moved earlier or later. Predecessor or successor tasks may be moved to allow the task to remain on its finish date.
Work Between Two flags joined together, each above a blue dot Constrains a task to start and finish on specific dates. The task becomes critical as it cannot be moved at all, so it cannot be delayed. Following a reschedule, the task is not moved at all.
Holding Pin A green pin If you apply a Holding Pin to a task, the task can be moved to a later date but cannot be moved to an earlier date when you reschedule the project, and if the task is moved, the pin moves with the task. The main purpose of the Holding Pin constraint is to hold unlinked tasks in place in a schedule.

Applying a constraint flag to a task

In this project, you will move the Dig Footings task to a different date then apply a Start On or After constraint to it, to show that it cannot start before the date to which you have moved it:

  1. Click and drag the Dig Footings task to the right on its bar so that its start and finish dates are approximately one week later than they were:

    The Dig Footings task being dragged to the right on its bar
  2. Right-click the Dig Footings task and select Constraint Flag - Start On or After from the menu that appears:

    Right-clicking the Dig Footings task and selecting Constraint Flag - Start On or After from the menu that appears

    A Start On or After constraint flag appears at the start of the Dig Footings task:

    A Start On or After constraint flag at the start of the Dig Footings task

Note that as you have moved the Dig Footings task one week later, the links that finish and start at the task are no longer vertical. If the Dig Footings task was not constrained to start or after its current date, rescheduling the project would move the task back in time one week and straighten the links, but as you have applied a constraint to the task, it will not be moved backwards in time when you reschedule. Reschedule the project now, noting that the Dig Footings task does not move, and that its successor tasks are moved one week forward in time to reflect its new start and finish dates.

Your project should now look something like this (note that the dates and durations of tasks may be different):

Illustration of the project at this stage

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Changing the start date of a project

Structuring a project using summary tasks