And so using these d's instead of deltas, this was Leibniz's way of saying, hey, what happens if my changes in, say, x become close to zero? So this idea, this is known as sometimes differential notation, Leibniz's notation, is instead of just change in y over change in x, super small changes in y for a super small change in x, especially as the change in x approaches zero. And as you will see, that is how we will calculate the derivative. Now there's other notations. If this curve is described as y is equal to f of x, the slope of the tangent line at that point could be denoted as equaling f prime of x one. So this notation, it takes a little bit of time getting used to, the Lagrange notation. It's saying f prime is representing the derivative.