FIG. 4: Schematic representation of (a) left-handed chiral CDWs and (b) right-handed chiral CDWs in a TiSe$_{2}$ CDW unit cell in real space and (c) typical 2D CDW. Charge concentration is indicated by the deeply coloured part. The colours correspond to the coloured nesting vector in the inset. In a CDW unit cell, the density peaks of three CDW are shifted at intervals of 2 c$_{0}$/ 3. If we look at one layer, the different intensities of three CDWs form a low symmetry structure. There are two phases caused by difference in stacking direction (red-blue-green or red-green-blue). As with cholesteric liquid crystals, the stacking results in the helicity of CDW systems. There is a mirror image relation between the two configurations in (a) and (b). As well as the stacking of CDWs layers in real space, CDW helicity defined in reciprocal space, H$_{CDW}$ ≡ q 1 · ( q 2 × q 3 ) reverses by space inversion ( q i → ( − q i )). H$_{CDW}$ is positive in a right-handed chiral CDW, and χ$_{CDW}$ is negative in a left-handed chiral CDW. 12