A characterization theorem is given for a class of developmental languages. The theorem binds together the number of occurrences of letters in the words of the given language with the distribution of these letters.
Ehrenfeucht, Andrzej and Rozenberg, Grzegorz, "The Number of Occurrences of Letters Versus Their Distribution in EOL Languages ; CU-CS-030-73" (1973). Computer Science Technical Reports. 29.