M - Commands  
MIRROR (command)
   Overview   Operations  Commands 

Creates a mirrored copy of objects

 Modify toolbar: 

 Command line: MIRROR

Select objects: Use an object selection method and press ENTER

Specify first point of mirror line: Specify a point

Specify second point of mirror line: Specify a point

The two specified points become the endpoints of a line about which the selected objects are mirrored.

Mirror a copy of the selected objects ? [Yes/No] : Enter  Y or  N or press ENTER

No

It places the mirrored image into the drawing and erases the original objects.

Yes

It places the mirrored image into the drawing and retains the original objects.

To mirror text as well, also see MIRRTEXT