Instance Mirror
Create > Instance Mirror
 

Description:

    The Instance Mirror command (Create > Instance Mirror) will create an instance of the selected object that is mirrored according to the settings in the instance mirror options dialog (Create > Instance Mirror > ...).  This is useful when you want to create a bi-laterally symmetrical object, but you want to be able to see both sides of the object update automatically when you change one.  Such examples of this might be when you are creating a face or a character.  The mirrored side is a separate object from the original side, and must either be de-instantiated (Create > De-Instantiate Instance) and merged with the original half (using Modify > Merge Objects followed by Modify > Merge Vertices), or be deleted and replaced by a call to the regular mirror command (Modify > Mirror) which will create regular editable geometry as part of the same object as the original half.
    When you are working with an object and its instance mirror, you often want open edges (edges with only one face attached) along the seam between the two objects to stay in the seam plane.  You can enable this behavior with the Enable Mirror Seam Preserve command (Selection > Enable Mirror Seam Preserve).  When enabled, any open edges that are within the tolerance set in the Instance Mirror Options dialog will be automatically pulled in to close the seam between the object and its instance mirror.


Notes:

- Instances cannot be edited in vertex, edge, or face modes.  You must edit the original half of the face to make changes on these levels, and they will be automatically updated on the instanced side.

- See also the Mirror command.