In this article, we'll explore how creating schedule dependencies (linking) work and how it can be an excellent tool for you to run your job with less effort and with better results.

Project plans usually require tasks to be performed in a specific order. For instance, a publication must be written and proofread before it can be printed. To achieve this, the Gantt application lets you link tasks so that they depend on each other.

By default, phases are usually linked in a 'Finish to Start' (FS) relationship (dependency), which means that the first task you select (the predecessor task) must end before the next task you select (the successor task) can start, and so on.

In BuildTools the linking is represented on the Gantt chart by lines with arrowheads joining each task to its successor. The arrow indicates the direction of the link: it goes from predecessor to the successor

predecessors
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A phase can have more than one predecessor. In this case its start date is determined by the predecessor link that gives it the latest start date. Ad date and times change during the course of the project the predecessor link that determines the start date of the task may also change.

multiple predecessors

Similarly a phase can have several successors. In this case the task determines the start date of all its successor tasks

Linking of phases that makes using the schedule so powerful: you can change the duration of one or more tasks, add a task, or remove a task from the a chain of linked phases, and all the dates are recalculated automatically so as to maintain the phase dependencies you have defined.

There are four possible linking types (dependencies) between phases

Finish to Start (FS) - the default: The phase cannon start before its predecessor ends, although it may start later. This is the most common type of relationship and will look like the below image.

Start to Start (SS): The phase cannot start until the predecessor starts, although it may start later.

Finish to Finish (FF): The task cannot end before the predecessor ends, although it may end later.

Start to Finish (SF): The Task cannot end before the predecessor starts, although, it may end later. This task relationship is rarely used.

Lets take a look at this simple example to help explain these concepts The following project plans illustrates all these relationships at work.

The writing phase follows on directly from the planning task. If the end date of planing changes, so will the start of the writing phase. This is typical Finish to Start relationship.

The  editing phase starts at the halfway point of the writing task. At this time the writers and the editors work together as a team until the copy text is complete  Their tasks effectively end at the same time. If the end date of writing changes, so will the end date of editing. The is a Finish to Finish relationship

The creation of artwork phase depends on the start of writing phase. In this case a lag has been introduced so that it doesn't start exactly when the writing starts. Even so, if the start of writing changes, so will the start of artwork creation. This is a Start to Start relationship.

The  final changes phase depends on the end of artwork creation, but an overlap is included so that it actually begins two working days before the end date of artwork creation. If artwork creation is delayed, so will be the start of final changes. This is the typical Finish to Start relationship.

Printing starts after final changes are complete. This is also a Finish to Start relationship.

The transferring of paper from the warehouse to the printers must be complete in order for printing to start, but is not related to any of the preceding tasks. The printers have limited storage capacity and do not want the paper to arrive until it is needed ('just in time' or JIT scheduling), so the start of printing drives the delivery of paper. If the start of printing changes for some reason, so will the end date for getting paper to the printers. This is a Start to Finish relationship.

This simple example only has single predecessors which is the simplest arrangement. However, a phase can have several predecessors. If that is you scenario be sure to think carefully about possible undesired consequences.

When linking tasks you can add a lead or lag time to extend a link backwards or forwards so that the successor task starts earlier or later than it otherwise would. For a default 'Finish to Start' link, this either introduces an overlap (lead time), so that the successor task starts before its predecessor ends, or it introduces a delay (lag time) that makes the successor task start some time after its predecessor ends.

For example when your building a new home and you know you need to have your window delivery scheduled two weeks in advance of when they are needed on site for installation. This might be a good time make use of a lead time for window delivery to ensure the window delivery is schedule at the right time.

You'll see that the Schedule Window Delivery is a successor of the Roofing Dry In phase. In this case we're using a Finish to Finish relationship but have added a 14 day lead time.

The advantage of this is that if the Roofing Dry In phase gets pushed back the Schedule Window Delivery phase will update accordingly and and maintain that two week lead period.

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