What type of constraint would you use if you want two lines to never cross?

To ensure that two lines do not cross each other, the best approach is to use the parallel constraint. This type of constraint enforces that two lines remain parallel to each other, effectively preventing them from intersecting at any point. By maintaining a consistent distance between the two lines, any alteration to one line will also adjust the other in a way that preserves their parallel relationship, thus ensuring they will never cross.

In Autodesk Inventor, creating a parallel constraint between two lines helps maintain their angular relationship and alignment throughout design modifications. The other constraints listed serve different purposes: for instance, the vertical constraint aligns a line vertically, the tangent constraint connects a line with an arc at a single point, and the perpendicular constraint forces two lines to meet at a right angle. None of these would inherently prevent the lines from crossing, which is why they are not suitable for this specific requirement of preventing intersection.