To Apply a Tangent 2D Constraint

Select the Tangent 2D Constraint tool.
Select a linear or non-linear (elliptical or circular) segment.
Select a non-linear segment if you have selected a linear segment in the first step (at least one of the selection must be a curve).
The two selected segments are now tangent to each other.

Note: For two segments to be tangent, they must be on the same plane.