On the transitivity of perspectivity in continuous geometries
Israel Halperin · Transactions of the American Mathematical Society · 1938
If 6 is a zero element of L, that is, if oà 6 holds for every a in L, then our independence over 0 is precisely the notion of independence as used in CG., part I, chap.2. * The Axioms I, II, III, IV hold in L(a) and in L(a, b).L(a) has a unit (greatest) element, namely a, and L(a, b) has a unit element b and a zero (smallest) element a.