sentences:  For case 3, where bits have changed in both the information and the check sections, notice that the number of zeros in the information section has "gone up", as described for case 1, and the binary value stored in the check portion has "gone down", as described for case 2.Therefore, there is no chance that the two will end up mutating in such a way as to become a different valid code word.A similar analysis can be performed, and is perfectly valid, in the case where the only errors that occur are that some 0-valued bits change to 1.