The simple linear relation between Δ (the gap between 4T2 and 2E zero-phonon lines) and parameters Dq, B. C and ΔE, (Stoke s shift), was discovered by analysis and calculations on the basis of Tanabe-Sugano matrices. The applicability of the relationship was explained on sone examples. The crystal-field strength criterion made by P. T. Kenyon was deduced and its limitation in the application was pointed out.