The brightness and contrast ratio of bistable twisted nematic displays are studied using the parameter space approach. By plotting the contrast ratio as a function of the twist angle and birefringence of the display, regions where the bistable twisted nematic display shows the best contrast ratio are clearly shown. We also studied the switching mechanisms of such displays. 1. Introduction Recently, a new class of twisted nematic (TN) liquid crystal displays (LCD) where the field-off state can exhibit bistability has been demonstrated 1-4 . Such bistable TN (BTN) displays has the advantage that, unlike ordinary LCD, no holding voltage is needed to display the images. At zero volts, the alignment of the BTN can take on either of two twist states, due to a proper choice of the rubbing directions of the alignment layers and the d/p ratio, where d, p are the thickness and pitch of the LC cell respectively. If the rubbing of the alignment layers favors a twist angle of f, and if the natu...