Help your students understand WHY there's a difference between a slope of zero and an undefined slope with this foldable. If your students understand the reason a vertical line has an undefined slope, they will be much less likely to confuse the two. This will definitely bring out one of those "ah ha!" moments in your classroom. If you print this using the 2-sided feature on your printer, the inside and outside of the foldable will line up perfectly. I've also included practice problems for determining the slope using the slope formula, finding the rise and run, and graphing lines with a given slope.