Principal parameters of KH2(1X)D2XPO4: the optical absorption and the refractive indices dispersion in a wide range, important for frequency conversion of iodine laser radiation are investigated. The use of "skew" PC elements is proposed. Tuning curves have been calculated for "normal" and "skew" FC elements . The doubling and tripling "skew" FC elements are used in large iodine laser installations: "Iskra-IV" (Russia) and "Perun" (Czech).