= Solution
A <cholesteric liquid crystal> is locally nematic but its <nematic director> rotates helically, for example
$$
\mathbf n(z)=(\cos q_0z,\sin q_0z,0),\qquad p=2\pi/|q_0|.
$$
The sign of $q_0$ specifies handedness and $p$ is the full-turn pitch. The sketch shows rods rotating through transverse planes. \b[This is helical orientational order, not a smectic density-layer lattice.] Although an apolar director repeats physically after a half-turn, the conventional pitch denotes a full $2\pi$ turn. A pitch comparable to visible wavelengths can give selective reflection.
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