Hysteresis effects. These occur almost invariably in physical adsorption on porous substances. the desorption curve always lies to the left and above the adsorption curve: i.e., at a given value of P/P0 more vapor is condensed in the pores if a specified pressure has been approached from a higher level than from a lower level. In some cases supersaturation may be required before condensation can begin, or the contract angle may change after being in contract with adsorbed vapors, but the principal explanation lies in the complex pore geometry of typical porous materials. As one simple example, if pores consisted of cylindrically shaped capillaries open at both ends, adsorption would begin on a surface curved in only two dimensions and pores would thus become filled in the adsorption branch when P/P0 reached the value given by