We next describe stages beneath and above a dividing wall. A stage directly above of a wall, shown in Figure , has two instead of just one vapor stream entering, one from the left of the dividing wall and one from the right. The outgoing liquid stream is split into two streams, one going to the left of the dividing wall and one to the right. Both outgoing liquid streams will have the same concentrations. For convenience we model the two streams with two concentration vectors but we add the equations