The optical properties of a chiral liquid crystal (CiLC) are analyzed in terms of the rotation of the principal axes (RP) together with the one-dimensional periodicity (DP) along the helicoidal axis. Within a general coupled mode formalism, the helicoidal nature of a CiLC is represented by both RP and DP. The formalism developed here is a powerful tool for calculating the optical properties of a CiLC for oblique incidence. Our numerical calculations show an exact agreement with the previous analytic results.