# Geometry: partition of a (0,1) -polytope, why is it adjacent?

There is one thing that I need to understand in this document:
Let's call $$P$$ a (0,1) -polytope, the convex hull of the vectors in $$X$$, where : $$X_i ^ 0 = {x in X: x_i = 0 }$$ Y $$X ^ 1_i = {x in X: x_i = 0 }$$ is an appropriate partition of $$X$$.
As you show that $$X_i ^ 0$$ is adjacent to $$P$$ at least one element of $$X ^ 1_i$$?