Parallel/Tangent

This constraint forces two vectors to be parallel.

In 2D (i.e., when a workplane is active), a zero-degree angle constraint is equivalent to a parallel constraint. In 3D, it is not.

Given a unit vector A, and some angle theta, there are in general infinitely many unit vectors that make an angle theta with A. (For example, if we are given the vector (1, 0, 0), then (0, 1, 0), (0, 0, 1), and many other unit vectors all make a ninety-degree angle with A.) But this is not true for theta = 0; in that case, there are only two, A and -A.

This means that while a normal 3d angle constraint will restrict only one degree of freedom, a 3d parallel constraint restricts two degrees of freedom.

This constraint can also force a line to be tangent to a curve, or force two curves (for example, a circle and a cubic) to be tangent to each other. In order to do this, the two curves must already share an endpoint; this would usually be achieved with a point-coincident constraint. The constraint will force them to also be tangent at that point.