| In Leibniz's notation, such an infinitesimal change in x is denoted by dx,
| and the derivative of y with respect to x is written
| suggesting the ratio of two infinitesimal quantities.
| (The above expression is read as "the derivative of y with respect to x",
| "d y by d x", or "d y over d x".
| The oral form "d y d x" is often used conversationally,
| although it may lead to confusion.)