TO MIRROR ARMATURE BONES ON X AXIS

1. Assume that we are making a human rig in standing position.
2. Set the view to front view.
3. Create the armature with the center bones along the z axis and the hand and leg bones on the positive side of the x axis.
4.Select the head of the root bone. Position 3D cursor at the head of the root bone pressing Shift+S >"Cursor to selected". Next change the pivot point to 3D cursor.
5. To mirror the bones on the Positive side of x axis to the negative side of x axis, select all bones on positive side of x axis,taking care not to select the central bones. Press Shift+D to duplicate, then Ctrl+m, x , -1 to mirror on the x axis.
6. Alternately, after duplicating the bones, press s, x -1 to mirror on the x axis.