1. Use a triangle drawn beneath a line to define the slope of the line. Drop a line down for a short distance from any point along the graphed line. From this point, draw another
http://www.ehow.com/how_8410543_slopelinesforms...

Perpendicular Lines are two lines that intersect to form right
http://www.chacha.com/question/whattwolinesinte...

I think the answer is a coordinate plane
http://answers.yahoo.com/question/index?qid=200908...
