If : L L is a bijection from the set of lines of a linear space (P, L) onto the set of lines of a linear space (P , L ) (dim (P, L), dim (P , L ) 3), such that intersecting lines go over to intersecting lines in both directions, then is arising from a collineation of (P, L) onto (P , L ) or a collineation of (P, L) onto the dual linear space of (P , L ). However, the second possibility can only occur when (P, L) and (P , L ) are 3