A large number of transient low mass X-ray binaries (LMXB) exhibit a knee in the decay stage of their light curves where the X-ray flux transits from a slow-decay stage to a rapid-decay stage. For the case of neutron stars, it had long been suggested that this sharp drop in the luminosity is due to transition from accretion to the propeller stage i.e.\ formation of a centrifugal barrier at low mass flux. Alternatively, it had been suggested that the knee is due to the ionization instability in the disk modulated by irradiation from the central source, a view which can address the knee in the lightcurve of both neutron star LMXBs and the galactic black hole transients (GBHT). In order to test the latter hypothesis, we have compared the dimensionless light curves of several BH and NS LMXB systems where the X-ray flux of each system is normalized with the knee flux and time is normalized with the outburst duration. The slopes of the exponential decay part of the dimensionless light curves of BH systems are much steeper than NS systems, implying that the underlying cause of the knee could be different in these systems.