Use the Predecessors tab in the Task Information dialog box to enter, review, or change predecessor information about the selected task. You can:
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Enter a predecessor for the current task.
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Click the predecessor type: Finish-To-Start (the default), Start-To-Start, Finish-To-Finish, and Start-To-Finish.
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Enter a lag time indicating a delay between linked tasks.
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Enter lead time indicating overlap between tasks.
Dialog box location
In any task view, click a task and then click Task Information 
Details
General task information
Name Shows the name of the selected task. You can enter another task name in the Name box to change it.
Duration Specifies the amount of time needed to complete the selected task. You can enter duration in minutes, hours, days, weeks, or months. To enter an estimated duration, type a question mark after the duration unit, or select the Estimated check box.
Estimated Specifies that the selected task has an estimated or tentative duration. Estimated durations are indicated by a question mark (?) after the time unit in a duration field.
Predecessors table
ID Type the identification number of the task's predecessor. Type the ID of only one predecessor per row and enter information about that predecessor in the remainder of the row. If you don't know the ID, you can click the name of the task in the Task Name field. You can only enter the ID of a task that already exists. If you want to link to a task in another project, with the other project open, you can type the project name and the task ID, for example, Project1\8.
Note: When you link tasks in different projects that are stored in Microsoft Office Project Server, the predecessor's task ID must be formatted correctly, as follows: <>\projectone.published\42. In this example, the angle brackets (<>) contain no information, projectone.published is the name of the project containing the predecessor, and 42 is the task ID of the predecessor task.
Task Name Click the name of the predecessor task from the list. All other tasks in the project are listed. If you typed an ID in the ID column, Microsoft Office Project automatically enters the task name in this column.
Type Specify a task dependency. You can click the dependency type from the list. Or you can type it, using its two-letter abbreviation.
Dependency types and abbreviations
Finish-To-Start (FS) The successor task cannot start until the predecessor task finishes. This is the default task dependency.
Start-To-Start (SS) The successor task cannot start until the predecessor task starts.
Finish-To-Finish (FF) The successor task cannot finish until the predecessor task finishes.
Start-To-Finish (SF) The successor task cannot finish until the predecessor task starts.
None No task dependency is set for this predecessor task.
Lag Type the lead time or lag time between the tasks.
Enter lead and lag time
The following are methods for entering lead and lag time.
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For lead time of a specified task duration, enter a positive duration amount including the time unit abbreviation, for example, 2d or 4w. If the successor uses a task calendar, lead time duration is based on that calendar; otherwise the duration is based on the project calendar.
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For lag time of a specified task duration, enter a negative duration amount, for example, -8h or -1mo. If a successor uses a task calendar, lag time duration is based on that calendar; otherwise the duration is based on the project calendar.
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For lead or lag time that's a percentage of the predecessor's task duration, enter a positive or negative percentage, for example, 20% or -10%.
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For lead or lag time that represents elapsed time rather than working time, enter a positive or negative elapsed duration amount, for example, 2ed or -2ew.
Note: You cannot create external predecessors and external successors with projects that are saved to a workspace.
