Spectra of carbon dwarf and giant stars may look very similar, as is demonstrated, e. g., by TT CVn and G77-61. Model calculations show that the bands of C2, CN and CO cannot be used in the range log g = 0……6 for log g determinations, since they are affected by abundance. If, moreover, lines of two ionization stages cannot be observed in any element, the only log g indicator would seem to be the ratio of the MgH band and the Mg I b lines. Simple analytic expressions are derived which demonstrate the relation between spectral appearance, abundances and gravity.