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

Let BB be a building over a type set II. Suppose J⊆IJ⊆I meets every connected component of the diagram of BB and let (P,L)(P,L) be the point-line truncation of the JJ-Grassmann geometry of BB. We present a theorem that allows one to identify shadows of apartments of residues of BB in the point-collinearity graph of (P,L)(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 BB is a convex subspace of (P,L)(P,L).