Variation in equivalent width ratio r′ = W′(C iλ1656)/ W′(C iλ1560) with C_f for several values of the C iλ1560 opacity (indicated in the upper right of the plot) and assuming x = 0.5. Blue and green lines correspond to C iλ1656 or C iλ1560, respectively, coinciding with the quasar emission line. The part of the plot for which C_f<C_f,min, assuming C_c = 1.0, is unphysical (dotted lines). Thick portions of lines correspond to values of r′ that can be exclusively obtained as a result of non-uniform covering.