when the command prompt requests a point to use this modifier.Click Perpendicular to when the command prompt requests a point to use this modifier.
PERPENDICULAR TO locks the rubber-band cursor perpendicular to (at right angles to) an existing reference point . You can use PERPENDICULAR TO whenever CC3 prompts you for a point. The reference entity can be a line, text, polygon side, spline, and so on
Usually you place at least one point before selecting PERPENDICULAR TO (or pressing F12. The prompt reads "select reference entity:".
Select a point on the reference entity. CC3 returns to the command you were using, but now the rubber-band cursor locks at the angle of the reference entity plus (or minus) 90°. The prompt changes to "Select point:";
Stretch the cursor in either direction until it looks the way you want, then select the next point for your command.
If you need to draw a building at an angle to the screen, you could consider using an aligned grid instead.
Text equivalent: PRP (or press function key F12)