In this paper, we analyse the ＇capillary pressure＇ curves obtained by the centrifuge method in order to perform the USBM wettability test. The physical displacement mechanisms present both in the porous plate and in the centrifuge method, are described for different cases of wettability of the pore surface. The wetting fluid is defined as the fluid being at the lower pressure while displacing the other fluid, and this displacement is defined as imbibition. On the other hand, the process in which the fluid under the lower pressure is the displaced fluid is defined as drainage. The capillary pressure is defined as the positive pressure difference between the two fluids. By adhering to these definitions, there is a unique and consistent terminology for the same physical process: the displacement of oil by water in an oil wet system and the displacement of water by oil in an water wet system are bath designated as drainage. An important result is that the centrifuge method is limited to the determination of drainage capillary pressure curves for strongly oil or water wet samples. There is no capillary equilibrium possible when a water wet sample is centrifuged under water because the wetting phase is under higher pressure than the nonwetting phase; the resulting ＇forced imbibition＇ curve should not be called a capillary pressure curve. For samples with ＇bicontinuous fractional wettability＇, the curves obtained by the centrifuge method correspond to ＇combination displacement＇, i.e, a combination of equilibrium drainage and forced imbibition coupled with blob mobilisation.