3D CUBI allows you to create a 3-D cubic parametric spline
(true 3-D curve) with nodes that do not lie in the same
plane (e.g., a spring).

Indicate a series of nodes through which the spline is
created, using Position Menu options.

A temporary marker (*) is drawn at the start of the spline
where the vector is to be established.

Indicate the start and end condition for the spline by
choosing from the following options:

NATURAL - to automatically assign a curvature of zero to 
          the appropriate end of the spline.

2 PTS   - to specify a tangent vector at a spline endpoint
          by indicating two points (positions).

ENDENT  - to determine a tangent vector using the tangency
          at the end of an existing curve, polyline, or 
          polygon entity.

TANGENT - to enter the x, y, and z vector components of 
          the desired tangent vector.
