Another generalization of the concept of a transversal would be a set that just has a non-empty intersection with each member of "C". An example of the latter would be a Bernstein set, which is defined as a set that has a non-empty intersection with each set of "C", but contains no set of "C", where "C" is the collection of all perfect sets of a topological Polish space. As another example, let "C" consist of all the lines of a projective plane, then a blocking set in this plane is a set of points which intersects each line but contains no line.