The only dimension used in the arrow diagram is task time. As with the Gantt chart, days or weeks are measured horizontally. Reading from the Gantt chart, you should identify each task with no predecessor. Lay a line for each one, beginning at Day zero, extending it to the right for the required number of days. The arrow does not need to be horizontal. Its horizontal component, however, must equal the task duration of the represented task.
The first task(s) should be laid out beginning about halfway up the vertical side of the page: One may extend diagonally upward, another diagonally downward (see Figure 7-3). Others will follow. This layout spreads out the arrow diagram, which is visually useful as a managing tool.
Task X has no predecessor and requires seven days. Note: seven days is the horizontal component.
Task Y has no predecessor and takes four days. Note: four days is the horizontal component attributable to Task Y.
Task Z has no predecessors and takes 10 days.
Tasks X, Y, and Z are the only tasks shown on the Gantt chart that have no predecessors. After completing the layout of tasks with no predecessor, you should pick the one with the shortest time duration Task Y and beginning at its endpoint, lay out a line for Task C, which is the next task. Note that Task C is a two-day task.
Next, look at the task endpoints on the chart and identify the closest endpoint to the project's start date. This endpoint is at the completion of Task C. It is six days from the start. Identify any tasks that are dependent upon the completion of Task C. Task D is dependent upon the completion of Task C and also is dependent upon the completion of Task X.
Task D will start as soon as Task X is finished. A dotted line from the end of Task C to the end of Task X, where Task D starts, shows the dependence of Task D upon Task C. It is shown as a dotted line, which represents slack. The slack applies to the Task String Y-C. It is slack that can protect either task or both from overruns that could impact the critical path.
Task D's endpoint occurs beyond Task Z's endpoint. Therefore, the next task you need to consider is any task that depends upon Task Z as its predecessor.
At a task terminal point, its successor task begins and is laid out. With several predecessors, this task starts at the end of the one extending farthest to the right. Other tasks then are connected to this starting point by dotted lines, indicating dependence. (The dotted lines also represent slack.)