Characterization of some subgraphs of point-collinearity graphs of building geometries

Let B be a building over a type set I . Suppose J ⊆ I meets every connected component of the diagram of B and let ( P , L ) be the point-line truncation of the J -Grassmann geometry of B . We present a theorem that allows one to identify shadows of apartments of residues of B in the point-collinearity graph of ( P , L ) . Also, other results related to shadows of apartments of residues and shadows of residues are presented, including a proof of the fact that the shadow of every residue of B is a convex subspace of ( P , L ) .