If V is the set of vertices of a simplex then H(V) is the simplex itself. For each of the 2n+1 subsets v of V the simplex H(v) is a subsimplex of H(V). That includes the −1-simplex which is the null set (Ø) which is spanned by Ø. The null simplex will not much concern us. The subsimplexes with the same number of vertices are generally considered together in proofs.
See Wolfram’s Simplexes.
Within an n-simplex H(V) there is a notion of two sub simplexes opposite each other. If v1 ∩ v2 = Ø and v1 ∪ v2 = V then H(v1) and H(v2) are opposite sub-simplexes. For instance in a triangle there is the side opposite a vertex and vice-versa. In a tetrahedron there is a face opposite a vertex, an edge opposite another edge.
There is a definition something like:
For a given n-simplex and pair, B and C, of opposite sub-simplexes, the set of rays starting in B and passing thru C intersect a large concentric sphere in a set whose measure is the ‘angle’ at B. (This is the real definition.)
Generalized Gauss-Bonnet? Choosing a random point within a simplex, The the angles of a simplex. Barycentric coordinates for a simplex, An Application to Physics, Altitude of a regular simplex, Zone the interior of concrete boundary, Cleaner Math