Sometimes more than one tasks needs to be completed before a "new task"
begins.
In this case, open up the "new task" Properties Display
Switch to the Predecessors Tab (another word for dependencies).
Double-click on the 1st item in the grid in the Name column
and you'll see a combo box appear
From that list select the preceeding tasks, then you may double- click on following
grid rows to add
more preceeding tasks.
Note that the column to the right of the Name column, is called the Dependency Type...
and JCV
Gantt supports several different types of dependencies displays.