You can specify a baseline to create a mirror image using MIRROR command. The mirror image is symmetrical with the original one. So if you want to create a symmetrical objects, you only need to draw a half one, then create a full one using MIRROR command.
When creating mirror objects using MIRROR command, you should specify a mirror line around which the mirror object is created. In the 3D space, this mirror line must be perpendicular to the XY plane in which the mirror line is included. The mirror line can be determined through specifying two points, while you creating mirror objects, you can optionally choose to remove or retain the source entity as you need once the mirror objects are created. Like the picture shown below. Additionally, you should notice that mirror is only effective to such plans that parallel to XY plan of current UCS.
System variable MIRRTEXT affects texts created with MIRROR command. When MIRRTEXT is on, the copy of text object is displayed in mirroring way as other objects do. When MIRRTEXT is off, the direction of text objects remains unchanged. Displaying as follows:
By default, the MIRRTEXT is on. It affects text objects created with TEXT, ATTDER, METEXT command as well as attribute definition and variable attribute. Text as well as constant attributes that are included in an insert block are mirrored together with the block while mirroring.
To mirror objects
Choose Modify > Mirror.
Select the object, and then press Enter.
Specify the first point of the mirror line.
Specify the second point of the mirror line.
On the command line, if you want to retain the original objects, type N; if you want to delete the original objects, type Y.
Command line MIRROR
To mirror an object, select it (A), and then specify the first point (B) and second point (C) of the mirror line
Mirror in 3D
You can create a mirror image of selected objects in three dimensional space. You mirror the objects about a mirror plane that you define by either specifying three points; selecting an existing two dimensional planar object; aligning the plane parallel with the xy, yz, or xz plane of the current UCS; or aligning the plane with the current view. You can delete or retain the original objects.
To mirror an object about a three dimensional plane
Choose Modify > 3D Operation > Mirror 3D.
Select the objects, and then press Enter.
In the command line prompt, choose 3 Points, or press Enter to select the default.
Specify the first point on the mirror plane.
Specify the second and third points on the plane.
In the command line prompt, choose one of the following:
Command line MIRROR3D
Select the object to mirror (A), and then specify the first point (B), second point (C), and third point (D) defining the mirror plane
The resulting mirrored object
MIRROR: Creates a mirror image copy of objects
MIRROR3D: Creates a mirror image of objects about a plane
System Variables Reference
MIRRTEXT: Controls how the MIRROR command reflects text