1. First three joint axes
 are parallel.


3.Axes 1 and 2 are parallel,
 axes 3 and 4 intersect.


5.Axes 2 and 3 are parallel,
 3 and 4 intersect.


2.Axes 1 and 2 are parallel,
 3 and 4 are parallel.


4.Axes 2, 3, and 4 are parallel.


6.Axes 1 and 2 intersect
 2 and 3 are parallel.


Figure 5.1. Special 4-DOF structures.

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