Two straight lines are said to be perpendicular to each other if they intersect at right angle. The angle formed at the point of intersection of the two perpendicular lines is $90$ degrees. When a vertical line cuts a horizontal line then these two lines are said to be perpendicular to each other. The relation between the slopes of two perpendicular lines is that the product of the slopes of the two perpendicular lines is always $-1$. If slope or gradient of one of the lines is known to us then we can solve for the gradient of its perpendicular line by taking negative of the reciprocal of the slope given.