Developing Drawing Strategies: Part 2 - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 183

FiGuRE 5.18 The first corner post
4. At the Specify first point of mirror line: prompt, click the
Snap To Midpoint button on the Object Snap toolbar; then pause the
cursor over either of the vertical handrail lines.
When a feature is symmetrical like the deck, you can use the Midpoint
snap to mirror objects about the center line.
5. Move the cursor directly to the left or right, as shown in Figure 5.19,
to mirror the post; then click to execute the mirror; and then press
↵ to accept the No option for deleting the source object.
FiGuRE 5.19 The first deck posts are in place.