This Object Snap option lets you make lines perpendicular to an existing line. To use this Object Snap, activate it from the Object Snap menu. I will use the inclined line, as in the following figure, to show you how this snap works:
Figure 3.15: A line inclined to an unknown angle+633
Make this inclined line and then select the Line tool again and click anywhere above the line. Now, move your cursor along the line and at a point where the new line is perpendicular to the existing line, a Perpendicular snap will show up, as shown:
Figure 3.16: A Perpendicular snap making a line perpendicular to the inclined line
A Perpendicular snap is not often required and so it is recommended to keep it off most of the time and activate it only when required.