To Add Tangency
1. Select a single wireframe piece.
2. Click Constrain > Tangency > Add Tangency. Alternatively, right-click and click Add Tangency. A tangency constraint is applied to the wireframe piece. The default direction of the tangency constraint applied to the wireframe piece depends on the condition of the surfaces that share the selected wireframe piece.
If one surface is frozen and the other is not, then the frozen surface is the primary surface in the tangency condition, that is, the default direction points from the frozen surface to the one that is not frozen.
If both the surfaces are frozen or if both the surfaces are not frozen, then a symmetric tangency constraint is applied by default between the two surfaces.
In a primary-secondary tangency constraint, the head of the arrow of the tangency constraint points toward the follower.
If the tangency condition is satisfied, the arrow is displayed in yellow. Otherwise, the arrow is displayed in black.