Constrains two entities to maintain a point of tangency to each other, or to their extensions. (This command is equivalent to using the Tangent option of the GeomConstraint command.)
Note: The second entity that you select is moved to be tangent with the first entity.
Accessing the Command
command bar: gctangent
menu bar: Assembly  2D Constraints  Tangent
ribbon: Parametric  2D Constraints  Tangent
toolbar: 2D Constraints 
: GcTangent
Prompts you in the command bar:
Select first entity: (Select a line, polyline segment, circle, arc, polyarc, ellipse or elliptical arc)
Select second entity: (Select another circle, arc, ellipse or elliptical arc; or a line or linear polyline segment, if the first object was not a line or polyline segment. )
Command Options
Option  Command 
Select first entity 
Select a valid entity:

Select second entity 
Select a second entity:
The second entity is moved to be tangent to the first, even when they do not touch each other physically; see figure below. 
After you place the constraint, icons appear near the entities indicating the type of constraint. If you do not see them, then use the ConstraintBar command to display them. To remove a constraint, click the tiny x on the constraint bar:
Related Commands
GeomConstraint  applies geometric relationships between entities and/or points on entities.
Dimconstraint  applies a dimensional constraint to an entity or between constraint points on entities; converts associative dimensions to dynamic dimensions.
GcCoincident  applies a coincident geometrical constraint to two points or constrains a point to an entity.
GcCollinear  forces lines to be collinear.
GcConcentric  constrains the center points of circles, arcs, ellipses or elliptical arcs to coincide.
GcEqual  constrains arcs and circles to the same radius, or lines to the same length.
GcFix  constrains points and entities at a fixed position.
GcHorizontal  constrains lines or pairs of points to lie parallel to the Xaxis of the current coordinate system.
GcParallel  forces two lines to be parallel to each other.
GcPerpendicular  constrains two lines to lie perpendicularly to each other.
GcSmooth  forces a spline to maintain fluid geometric continuity with another spline, line, arc or polyline.
GcSymmetric  constrains two entities or points to lie symmetrically with respect to a selected line.
GcVertical  constrains lines or pairs of points to lie parallel to the Yaxis of the current coordinate system.
ConstraintBar  controls the display of the constraint bar of entities.
Delconstraint  removes all dimensional and geometrical constraints from an entity.