Figure 2

Cartoon of a portion of a lattice consisting of three types of molecules K, P, S at three different times $t=0,T,2T$. In the portion illustrated, $N=2$, so there are three layers, $n=0,1,2$, and $M=7$ horizontal sites. For simplicity it is assumed that it takes T seconds to etch a P and K molecule while S is not etched by this acid.