In what follows, we use the (change) notation instead of d (derivative notation) to recognize that

In what follows, we use the Δ (change) notation instead of d (derivative notation) to recognize that interest rate changes tend to be discrete rather then infinitesimally small. For example, in real-world financial markets the smallest observed rate change is usually one basis point, or 1/100 of 1 percent.

 LO.1