Makes an arc or circle tangent to another arc or circle, or to a line.
- Start with a line and an arc/circle. Activate the Tangent constraint and select either object (when one object is a line, the selection order does not matter).
- Select the other object.
The line is moved, in an offset direction, so that it is tangent to the arc/circle. The tangent constraint symbol is added at the point of tangency. 
- If you edit either object, the tangent constraint will be maintained.
If you apply this constraint to a line whose offset will not meet the arc/circle . . . 
. . . the line is moved so that its extension is tangent to the circle. 
You can also make two arcs/circles tangent to one another. In this case, the first selected object will move to meet the second selected object. If the arc does not meet the other arc/circle . . 
. . . the arc is moved so that its extension is tangent to the arc/circle. 