Is there a way to ensure that lines/vertices are parallel to each other? And a command that can confirm parallelity between lines/vertices? The goal is to make a-a’ and b-b’ planar and parallel to each other, then close the SubD model with Cap or FillSubD leading to a planar cap that contains no isocurves/divisions. (Working on Rhino7, the model is a SubD surface, the vertices’ control points are located on the same plane - the current Construction Plane). Advice would be greatly appreciated. Below are screenshots and the geometry.
Rhino Forum Question_orient lines to be parallel to each other.3dm (46.8 KB)
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the model comes in empty except your text and one point
I would do it by a workflow similar to:
start-situation
cplane
set the cplane to 3 points, and move the 4th point away randomly
subDPlane
draw a _subDPlane with 1x1
if you need a parallelogram additionally use _shear
stitch
that s the reason why I moved one point at the beginning:
now the selection for _stitch is much easier.
make it worth before stitch
… is a super nice workflow to more easily select the edges / vertices…(and overcome the limitations of this command regarding selection)
analyse
if you really need to measure wether they are parallel:
_angle will allow you to measure with 4 points.
(@theoutside it s a pity or even a bug, that _angle does not accept 2 SubD edges)
… but I think, if it is not parallel, you have to fix it - so above will make sure you have a cap and parallel edges.
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There would be if Rhino had geometrical constraints. 
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clever solution! well done-
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Thank you @theoutside for pointing out my error. For reference I have uploaded the file with the originally intended content.
Rhino Forum Question_orient lines to be parallel to each other02.3dm (122.8 KB)
Thank you @Tom_P ! That works really well. I was considering something similar using a 3-point rectangle and then manually move the SubD vertices to the rectangle´s corner points to ensure that the edges are parallel and planar to each other, followed by the Cap command. Upon reflection I realise, that when working with this SubD surface the edges themselves cannot, strictly speaking, be parallel to each other as the are not straight but slightly curved. What can be made parallel are ideal straight lines between the vertices that define the endpoint of the edges.
Brian James on the RhinoGuide Youtube channel has an excellent tutorial where he shows how to use the Gumball’s scale handle to quickly align control points so they sit in the the same plane (2m 30sec in). This, combined with the Orient command or similar, might be an alternative workflow. Here’s the video: https://youtu.be/qBkXs9PcpME?t=150
