The second parity rule of Chargaff (A≈T and G≈C within one strand) holds all 
over the living world with minor exceptions. It is maintained with higher 
accuracy for long sequences. The question addressed in the article is how 
different sequence types, with different biases from the parity, contribute to 
the general effect. It appears that the sequence segments with biases of 
opposite sign are intermingled, so that with sufficient sequence lengths the 
parity is established. The parity rule seems to be a cumulative result of a 
number of independent processes in the genome evolution, with the parity as 
their intrinsic property. Symmetrical appearance of simple repeats and of Alu 
sequences in the human DNA strands, and other contributions to the Chargaff 
parity II rule are discussed.