If you input an x into f prime, you're getting the slope of the tangent line at that point. Now another notation that you'll see less likely in a calculus class, but you might see in a physics class, is the notation y with a dot over it. So you could write this as y with a dot over it, which also denotes the derivative. You might also see y prime. This would be more common in a math class. Now as we march forward in our calculus adventure, we will build the tools to actually calculate these things. And if you're already familiar with limits, they will be very useful, as you can imagine, because we're really going to be taking the limit of our change in y over change in x as our change in x approaches zero.