Mirror Cut creates
a plane of symmetry that mirrors the selected object. You can position
the symmetry plane using the manipulator. Changes that you make
to the original object are then updated on the mirrored object.
This can make symmetrical modeling of certain types of objects much
easier (for example, creating an environment with streets, creating
a skateboard park).
Mesh > Mirror Cut > 
- Cut Along
-
Specifies the pair of axes to mirror the
object across: YZ, XZ, or XY.
- Merge with the original
-
Combines the original
object and the mirrored object into a single mesh. Subsequent changes that
you make to the original object are not updated on the mirrored
object as a result.
- Merge vertex threshold
-
Vertices within this
distance of each other will be merged. This option is only available
when the Merge with original setting is turned on.
