Enable Mirror Seam Preserve
Selection > Enable Mirror Seam Preserve
 

Description:

    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:

- You set the options for this functionality in the Instance Mirror options dialog.