Doc:2.6/Manual/3D interaction/Transformations/Advanced/Mirror
Mirror
Mode: Object and Edit modes
Hotkey: CtrlM
Menu: Object/Mesh » Mirror
Description
Mirroring an Object or Mesh selection will create a reversed version of the selection. The position of the mirrored version of the selection is determined by the Pivot Point. A common use of mirroring is to model half an object, duplicate it and then use the mirror transform to create a reversed version to complete the model. Note that mirrored duplicates can also be created with a Mirror modifier.
Read more about the Pivot Point »
Read more about the Mirror Modifier »
Usage
To mirror a selection along a particular global axis press:
CtrlM, followed by X, Y or Z.
The image Mirroring a selection shows the results of this action after a mesh element has been duplicated.
In Mesh mode, you can mirror the selection on the currently selected Transform Orientation by pressing the appropriate axis key a second time. For example, if the Transform Orientation is set to Normal, pressing:
CtrlM, followed by X and then X again
will mirror the selection along the X-axis of the Normal Orientation.
Read more about Transform Orientations »
You can alternatively hold the MMB 
to interactively mirror the object by moving the mouse in the direction of the mirror axis.
