Material preview pane context menu
These settings also appear on right-click context menus for the thumbnail previews and the thumbnail background.
Assign to Objects
Assigns the current material to selected objects.
Assign to Layers of Objects
Assigns the current material to the layers of selected objects.
Assign to Layers...
Assigns the current material to selected layers.
Select Object(s)
Select objects that use the current material.
Create New Material
Creates a new material from a template.
Save to File
Saves a material to a Rhino .rmtl file.
Copy
Copies the selected item to the Clipboard.
Paste
Creates a new item based on the contents of the Clipboard.
Paste as Instance
Creates a new item based on the contents of the Clipboard. The new item will be linked to the original item.
Delete
Deletes the selected item.
Select All
Select all items in the list.
So you can change the common settings and thumbnail properties of all items.
Reset to Defaults
Changes all of the item settings to the default.
Duplicate
Creates a new item from the selected item.
Tags...
Pops up the Render Content Tag dialog to manage tags. Tags group or categorize items to make searching easier.
Filter By Tags
Hides the items that don't have the selected tags.
Sort Alphabetically
Lists the items with their names in ascending (A to Z) order.
Remove Instancing
Removes the connection between instanced items.
Update Preview Now
Updates the preview of the selected item when auto-update is disabled.
Floating Preview
Floats the preview image in a resizable window.
Floating Editor
Edits the selected item in a separate dialog.
Thumbnails
Sphere
Cube
Cone
Pyramid
Torus
Plane
Scene
Show Labels
Displays thumbnail name labels when in Grid mode.
List mode always displays labels.
Show Units
Displays the dimension of the object in the preview.
The object dimension can be changed in Preview Properties.
Auto-Update Preview
Automatically updates all previews as settings change.
Update All Previews
Update previews manually when Auto-Update Preview is off.
Properties
Opens the Preview Properties dialog box.
Preview thumbnail images and the background area between thumbnail images both have context menus.
Preview Properties
The preview properties settings control the appearance of the thumbnail images.
Geometry
Specifies the shape of the object that displays the material in the preview thumbnail.
Sphere
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Pyramid
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Cube
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Torus
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Cone
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Plane
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Scene
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Size
Sets the dimension of the preview object in meters.
Background
Controls the appearance of the thumbnail background.
None
The default gray environment is used.
Checkered
A gray checkered backdrop is used.
Studio
Custom
Uses an environment as the background. 
Click to assign environment
Select an existing environment or create a new one.
Select Environment
The list of existing environments.
Create a new environment.
View
Controls the right-mouse drag object or camera rotation.
Rotate object
Right-mouse drag rotates the object.
Rotate camera
Right-mouse drag rotates the camera.
X angle
Sets the horizontal rotation angle for the object or camera.
Y angle
Sets the vertical rotation angle for the object or camera.
Lighting
Simple lighting
Uses the default lighting.
Skylighting
Uses the current skylighting settings.
Saves the thumbnail properties as the default settings.
Uses the default settings.
Close
Closes the large preview pane.