helix along the c-axis was achieved in the CDWs and the helicity difference results from the stacking sequence. This configuration can explain the low symmetry charge distribution in the STM mea- surement and the 2-fold symmetry in the optical measurement. In the STM measurement, the tunnelling current can obtain information about one or two atomic layers. In one atomic layer, the intensity of three CDWs differs due to the distance between the observed surface and each CDW peaks. A CDW whose peak was closer the surface was more clearly observed ( Iq$_{1}$ ) than a CDW that was further away ( Iq$_{3}$ ). In terms of the intensity difference between Iq$_{1}$ and Iq$_{3}$ , the FFT image obtained with STM (Fig. 2) reveals the direction of the heli- cal stacking. Along the positive direction of the c-axis, clockwise is the “left-handed” and anticlockwise is the “right-handed” state. The phase shift in one layer as in a conventional 2D CDW cannot explain only the 2-fold symmetry structure. Therefore, the optical results provide evidence for an inter-layer phase shift. To consider the origin of the helical stacking of CDW, we take account of the electronic structure of 1T-TiSe$_{2}$ in reciprocal space. In contrast to a typical 2D CDW system such as 1T-TaS$_{2}$, the nesting vectors of TiSe$_{2}$ originally have a c -axis component [9]. In conventional 2D CDW materials such as 1T-TaS$_{2}$, at first the in-plane triple- q nesting vectors forms a hexagonal superlattice structure in a S-Ta-S layer. Then, by the Coulomb interaction, the S- Ta-S layers stack with the next CDW layer so as no two density peaks have the same position (Fig. 4 (c)). The phase shift in the neighboring layer results from inter-layer coupling. On the other hand, in 1T-TiSe$_{2}$, the 3D nesting vectors, that originally has a c -axis component, induce a charge modulation directly along the nesting vectors. Accordingly, in 1T-TiSe$_{2}$ there is no conventional layer stacking as in 1T-TaS$_{2}$. Moreover, relative phase shifting along the c -axis between q 1 , q 2 , and q 3 can reduce the Coulomb interaction. As a result, the configuration achieves a helical structure (Fig. 4). From above discussion, we propose generalized CDW chirality in triple- q in reciprocal space. , where q 1 , q 2 , and q 3 are the nesting vectors. If the H$_{CDW}$ is zero (three nesting vectors in one atomic plane), a triple- q CDW exists simultaneously in one layer in a CDW unit cell. But if the H$_{CDW}$ is nonzero (inset in Fig. 4), there is a degree of freedom owing to charge helicity, that is, H$_{CDW}$ < 0 represents a left-handed chiral CDW and H$_{CDW}$ > 0 represents 6 H$_{CDW}$ ≡ q 1 · ( q 2 × q 3 ) (1)