The Stress Trajectories option in the Display Options dialog, displays the principal stress trajectories in the form of a long line for s1 and a short line for s3. The trajectories are placed and centered at every finite element node.
If a square dot < appears instead of the stress trajectory symbol, this indicates that conditions are hydrostatic (s1 = s3) at that point.
If you are viewing differential data the stress trajectories will show the orientation of the differential principal stresses at each node.
Stress Trajectories is also available in the toolbar.