Reverse twist, chiral nematics

The majority of current commercial liquid crystal displays are based on the twisted nematic electrooptic effect using active matrices to give, for example, complex computer lap-top screens [13]. It is often overlooked that these displays are based on long pitch chiral nematic materials to remove so-called reverse twist, and the original papers [254, 255] referred to the effect as a positive planar cholesteric to nematic phase... 

As seen from Fig. 96b, the switching process is unambiguous as regards the motion of n and P (sterically bound to n) on the upper side of the chevron, P rotates counterclockwise, on the lower side it rotates clockwise when we switch from 1 to 2 everything turns around in the reverse switching direction. This explains why there are no twist and antitwist domains like the ones observed in twisted nematics prior to the time when chiral dopants were added in order to promote a certain twist sense. 

---