Understanding Constraints

Constraints has impact on schedule thus it is important to understand them in detail. Broadly constraints can be divided in two main categories.

Flexible Constraints

Flexible Constraints do not have specific dates associated with them e.g.As Soon As Possible (ASAP) and As Late As Possible (ALAP).  Setting these constraints allows you to start tasks as early as possible or as late as possible with the task ending before the project finish, given other constraints and task dependencies in the schedule.

Inflexible constraints

Inflexible constraints have specific date associated with them e.g. Must Start On (MSO) and Must Finish On (MFO) require an associated date, which controls the start or finish date of the task. These type of constraints must be applied when you must take into account external factors, such as the availability of equipment or resources, deadlines, contract milestones, and start and finish dates


When you enter a new task in the Task Name field, Project assigns the As Soon As Possible (ASAP) constraint when you schedule your project from the start date. When scheduling from the finish date, Project assigns the As Late As Possible (ALAP) constraint.

If you drag the mouse in the chart portion of the Gantt Chart to create a new task, however, Project automatically assigns the Start No Earlier Than (SNET) constraint when you schedule your project from the start date. When scheduling from the finish date, Project automatically assigns the Start No Later Than (SNLT) constraint.

When you manually enter a start or finish date for task, you automatically apply a constraint to it.

When you schedule your project from the start date and you type a start or finish date for a task, Project constrains the task to begin or complete no earlier than that date. If you schedule your project from the finish date, however, Project constrains the task to begin or complete no later than the date you type.