Two lines are perpendicular if they meet each other at a right angle (90 degrees). For two lines to be perpendicular, their slopes need to be “negative reciprocals” of each other. Reciprocal means the inverse of a fraction – such as b/a instead of a/b. Negative reciprocal mean it also has the opposite sign of the other slope. To determine if two lines are perpendicular first put each line in slope intercept form, then investigate their slopes.