One's complement have a positive and a negative zero. Two's complement is the solution to such problem. Because of this, two's complement have more negative numbers than positives. (Just one more).

http://www.youtube.com/watch?v=9W67I2zzAfo&feature=related

The same number of bits are used to represent 1's complement and 2's complement. To take 2's complement, first take the 1's complement, then add 1 to the result.

What is 9's complement of 321?

Suppose A is a subset of S. Then the complement of subset A in S consists of all elements of S that are not in A.

The intersection of two sets A and B consists of all elements that are in A as well as in B.

1's complement numbers are those numbers which are obtain after converting 1 to 0 and 0 to 1.

for e.g.

110010

1's complement of this number is:001101

2's complement is obtain by adding 1 in 1's complement of number.

for e.g.

2's complement of above number:001101

+ 1

---------------

001110