Perpendicular means that two lines intersect at right angles. Like a “+” sign, two line with completely opposite slopes will cross at exactly 90º to each other. By “completely opposite slopes” it is meant the negative reciprocal of each other. For example, 3/4 and -4/3 are perpendicular slopes. It is interesting to notice that 3/4 x -4/3 = -1. The product of perpendicular slopes will always be -1.