# Relative interior of a polytope

Can anyone explain to me what is the idea of relative interior of a convex hull of a set of finite points ? For interior of a set , I understand that it is a set which excludes its boundary. Is interior of a set a subset of relative interior of a set ?

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Here is one example. Take a 3d point set, all points lie on a common plane $h$. The convex hull of this set is a "2d object" with affine hull is $h$. Its interior in $\mathbf{R}^3$ is $\emptyset$. The relative interior however is the convex hull without vertices and edges. In other words it is the interior w.r.t. the plane $h$.