A pair of straight lines can be parallel lines or perpendicular lines or neither. Two lines are said to be parallel when their slopes are equal. Two parallel lines never intersect each other. On the other hand two lines are perpendicular when the product of their slopes is equal to -1. Two perpendicular lines intersect only once and at right angles. So when you are given two lines and asked to find if they are parallel or perpendicular lines, first compute the slope of the lines. Identify two points on a line to compute its slope. The slope of a line is the ratio of the difference of the y coordinates to the difference of the x coordinates. If the slope of the two lines is equal, then they are parallel lines. In case the product of the lines is equal to -1, they are perpendicular lines. This video shows how to identify parallel and perpendicular lines.