(1.9) Let $\mathscr{X}$ be a Noetherian scheme, and let $D$ be an effective divisor on $\mathscr{X}$, with prime components $D_{i},\,i\in I$. A stratum of $D$ is a connected component of the schematic intersection $D_{J}=\cap_{j\in J}D_{j}$, for some non-empty subset $J$ of $I$. An open stratum is a stratum $S$ minus the union of the prime components of $D$ that do not contain $S$.