When lines and/or points are in perpendicular to one another, the perpendicularity line between them measures the distance between both points and/or lines.
So to measure the distance between point c and line AB, a perpendicular line has to be drawn from c to AB or from AB to c. Either of these will arrive at the same result.
It should also be noted that the angle at the point of intersection of perpendicular lines is 90°.
