The average ordering of colloidal particles in suspension changes during the process of reducing one spatial dimension, for example, when a finite volume of the suspension is squeezed between two plates until a quasi-two-dimensional system is produced. There are two reasons for this to happen : first, the interaction between particles is now also mediated, and hence modified, by the walls, and second, a spatial degree of freedom is hindered. Effects of the latter type, referred to as effects of the dimensionality, are studied in this work, from a theoretical point of view, for colloidal suspensions of charged and uncharged hard spheres. They are studied by comparing the structure of three- and two-dimensional systems without considering any modification in the interparticle pair potential. We show that the system may undergo phase transitions induced by the change in its dimensionality.