Let's call that our u-axis maybe. And I did u intentionally, because that's the variable we'll use in our proof of the chain rule video. And let's say this right over here is our function. Let's say this right over here, that is c. This right over here is u of c. u of c. And then let's just take some arbitrary x over here. So some arbitrary x, and then this right over here, this right over here is u of x. is u of x. So if we define, if we define our change, if we define, let me do this. If we define our change in, our change in u is equal to u of x minus u of c, which makes sense, because this is our change in u.