Choose Home tab→Relate group→Horizontal/Vertical 
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Do one of the following:
To make a line horizontal or vertical, click the line (A).
To make two key points horizontal or vertical, click a key point, and then click another key point.
Tip:
The line's current orientation determines how it is positioned after you select it. For example, if a line is closer to a horizontal orientation than a vertical orientation, it becomes horizontal.