  Projective properties are those preserved by projections. Take, for example, two planes Γ and H and a point P outside them. Let Φ be any figure on Γ. Draw straight lines from P through each point of Φ. The figure formed by the points where these lines meet H is the projection of Φ on H from P. Generally this figure will differ from Φ in size and shape. But the projection of any number of straight lines on Γ meeting each other at certain points generally consists of an equal number of straight lines on H meeting respectively at the projection of those points. What happens, however, if the straight line joining P with some point Q of Γ never meets H, because PQ happens to lie on a plane parallel to H? (See Figure 3.)