I have a triangle which reflects perfect symmetry of my 3d scan imported to Rhino. I try to do a cplane command to have perfect symmetry during modeling. I use the 3point option in cplane command after picking up triangle corners. After using cplane command all 4 views should have the main world axis aligned to that triangle but when I look at ortographic views they are never perfectly aligned to the triangle. Why? How to change that. I would like to have all 4 views perfectly aligned to that triangle. On the right and front view that triangle should be visible as horizontal which comes across the main world axis.
No. Wrong assumption.
Default 4 views command align your views to world, absolute XY plane.
Create your CPlane.
Maybe save it? You can use Named CPlanes tab…
Then use the CPlane views, not the red car, but the plane 
(another way could be to actually rotate and align your whole 3d model to have the actual XY plane as the symmetry plane… but that depends on what you are doing…)
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Default 4 views command align your views to world, absolute XY plane
Maybe I wrote it badly. I used cplane command, not a 4views command. I expected to see all 4 views set to that cplane not only perspective.
Changing preferences>modeling aids>Universal construction planes quite works. But one time it works and the other it doesn`t. I not solved it why sometimes all views are not aligned to cplane.
Thanks for that “Top View of cplane”. I haven’t known that.
Your document and all of its geometries are stored and saved in your 3dm with absolute coordinates, XYZ.
Normally you see the a XY grid in the top and perspective viewports, and orthogonal grids in the front and right viewports.
CPlanes will not edit or modify the coordinates of your geometries, your document will be unchanged, you will just create a construction plane, a different coordinate system to assist you creating new geometries (whose will be wrote in the document using again the absolute system).
You can see CPlanes as a “temporary” coordinate system.
(you can remove the “temporary” thing if you use the named CPlanes tab…)
When you define a CPlane, it is set only for the current viewport you are working.
All other viewports will have their default CPlanes.
You can save your cplane with named cplanes and “recall” your saved cplane in all other viewport.
But remember that using the “4 viewport” button will reset your current cplanes, and you would need to re-recall your named cplanes all over again.
Tricky.
Or you can stick and work into a single viewport and use the Plane (not the car) buttons.
Still tricky.
Really, just physically orient your geometries to the absolute XY plane. Much simpler.
Orient 3 points.
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