If 1 is added to the one's complement, the resulting number is two's complement of the original binary number. For example, lets take a number "0110", its 1's complement is "1001", add 1 and the result is "1010". The result "1010" is 2's complement of "0110". From the table below, note that "0110" represents +6, and "1010" represents -6.

Two's complement representation table...

       Decimal number                  Two's complement 
             0                                0000 
             1                                0001 
             2                                0010 
             3                                0011 
             4                                0100 
             5                                0101 
             6                                0110 
             7                                0111 
            -8                                1000
            -7                                1001
            -6                                1010
            -5                                1011
            -4                                1100
            -3                                1101
            -2                                1110
            -1                                1111
            -0                                 -