Diagrams and Incidence Structures

Abstract We show a purity assumption which seems to be implicit in the theory of the geometry of diagrams (developed by F. Buekenhout in: Geom. Dedicata 8 (1979), 253–257; 296–298; J. Combin. Theory Ser. A 27, (1979), 121–151); characterizations of those structures on which this assumption holds are given (pure structures), and a sufficent condition on a structure to be pure is also presented.