The projective linear groups therefore generalise the case PGL(2, C) of Möbius transformations (sometimes called the Möbius group), which acts on the projective line. Note that unlike the general linear group, which is generally defined axiomatically as "invertible functions preserving the linear (vector space) structure", the projective linear group is defined "constructively," as a quotient of the general linear group of the associated vector space, rather than axiomatically as "invertible functions preserving the projective linear structure". This is reflected in the notation: PGL("n", "F") is the group associated to GL("n", "F"), and is the projective linear group of ("n"−1)-dimensional projective space, not "n"-dimensional projective space. A related group is the collineation group, which is defined axiomatically. A collineation is an invertible (or more generally one-to-one) map which sends collinear points to collinear points. One can define a projective space axiomatically in terms of an incidence structure (a set of points "P," lines "L," and an incidence relation "I" specifying which points lie on which lines) satisfying certain axioms – an automorphism of a projective space thus defined then being an automorphism "f" of the set of points and an automorphism "g" of the set of lines, preserving the incidence relation, which is exactly a collineation of a space to itself. Projective linear transforms are collineations (planes in a vector space correspond to lines in the associated projective space, and linear transforms map planes to planes, so projective linear transforms map lines to lines), but in general not all collineations are projective linear transforms – PGL is in general a proper subgroup of the collineation group. Specifically, for "n" = 2 (a projective line), all points are collinear, so the collineation group is exactly the symmetric group of the points of the projective line, and except for F2 and F3 (where PGL is the full symmetric group), PGL is a proper subgroup of the full symmetric group on these points. For "n" ≥ 3, the collineation group is the projective semilinear group, PΓL – this is PGL, twisted by field automorphisms; formally, PΓL ≅ PGL ⋊ Gal("K"/"k"), where "k" is the prime field for "K;" this is the fundamental theorem of projective geometry.