So we say no f prime under, it's going to happen under three conditions. The first condition, you could say, well we have a vertical tangent. Vertical tangent. Why is a vertical tangent a place where it's hard to define our derivative? Well remember, our derivative, we're really trying to find our rate of change of y with respect to x. When you have a vertical tangent, you change your x a very small amount. You have an infinite change in y, either in the positive or the negative direction.