CPlane 

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CPlane command
Sets the origin and orientation of the construction plane in the active viewport.
A construction plane is like a tabletop that the cursor normally moves on. The construction plane has an origin, x and yaxes, and a grid. The construction plane can be set to any orientation, and each viewport’s construction plane is independent of those in other viewports.
The construction plane represents the local coordinate system for the viewport and can be different from the world coordinate system.
Rhino’s standard viewports come with construction planes that correspond to the viewport. The default Perspective viewport, however, uses the world Top construction plane, which is the same construction plane that is used in the Top viewport.
The construction plane grid lies on the construction plane. The dark red line represents the construction plane xaxis. The dark green line represents the construction plane yaxis. The red and green lines meet at the construction plane origin. The color of these lines can be changed.
To change the direction and origin of a construction plane, from the menu, use the CPlane command. Preset construction planes: World Top, Right, and Front give you quick access to common construction planes. In addition, you can save and restore named construction planes and import named construction planes from another Rhino file.
Coordinate input, elevator mode, object snaps , and other cursor constraints allow the cursor to move away from the construction plane.
Notes
Construction plane behavior in the viewports is controlled by the Standard and Universal options. With the Standard option, the construction plane of each viewport is independent from all of the other constructions planes. With the Universal option, the behavior of the construction planes in the viewports is linked. Set these in Modeling Aids Options.
The construction plane is infinite. The array of lines lying on a specified portion of the construction plane in the viewport is the grid. The grid is a visual reference only. The size, spacing, and color of the grid lines can be changed.
The x and yaxes of the construction plane are shown on the grid in color by default. The visibility and color of the grid axes can be changed.
Custom construction planes can be named and saved in the 3dm document, They can be restored in any viewport using the NamedCPlane command.
Pick a new origin point or choose an option.
Options
The construction planes in all viewports change so their origin is at the picked location.
Sets the construction plane perpendicular to a curve.
Steps
Select a curve to orient construction plane.
Drag the construction plane's new x and y axes to a new location.
Notes
Use the Curve option to quickly position your construction plane at various stations along a central rail curve as preparation for doing a one rail sweep, for example. Once your construction plane is positioned, you can use drawing tools to draw a crosssection curve in the 3 D location where it needs to be. This direct drawing method can be quicker and more convenient than drawing the curve on a world plane and then trying to move and rotate it into place in 3D.
The direction of the curve affects the direction of the axes. If the construction plane does not go the direction you want, flip the direction of the curve with the Flip or Dir commands.
Changes the elevation (zdirection) of the construction plane.
Steps
Pick a new elevation.
Or, type a distance to move the origin from its previous location.
Notes
Positive numbers move the construction plane up along the positive zaxis.
Negative numbers move the construction plane down.
Restores the next used construction plane.
Steps
Click Next or type N.
Sets the construction plane in the active viewport to an object.
Steps
Select a curve or surface.
The origin and tangency of the new construction plane is based on the "midpoint" of the untrimmed version of the surface.
Restores the previously used construction plane.
Steps
Click Previous or type P.
Rotates the construction plane.
Steps
Pick the start of the rotation axis.
Pick the end of the rotation axis.
Type the angle to rotate the construction plane.
Or, pick two points to specify the angle.
Sets the construction plane to match a surface. The placement is constrained so that the construction plane is tangent to the surface.
Steps
Select a surface.
Pick an origin or press Enter.
Pick an xaxis direction or press Enter.
Surface Options
Flip
Reverses the direction.
IgnoreTrims
Uses entire underlying surface.
Moves the construction plane origin to a selected point.
Steps
Pick a location to move the origin to.
Option: All.
Sets the construction plane in the active viewport parallel to the view.
Steps
Click View or type V.
Restores the specified default construction plane.
Steps
Specify an option: Back, Bottom, Front, Left, Right, Top.
Sets the construction plane in the active viewport by specifying points to establish the x, y, and zdirections.
Steps
Pick a new origin, or press Enter.
Pick the xaxis.
Pick the yaxis.
3Point Options
Sets the construction plane vertical to the current construction plane.
Sets the construction plane zaxis direction.