 The once complement format is the second way we came up with for representing integers in computers. It will still allow us to represent both positive and negative numbers, but the format's a little more complicated than the sign and magnitude method. In this case, we'll still use our leftmost bit as a sign bit effectively, but we're not going to explicitly set it as such. Instead, if we've got a positive number, we'll just write down that number as we'd expect. It will have lots of zeros on the left-hand side because those don't actually change the value of my number. But when I want a negative number, I would take that positive representation and flip all of the bits. So any zeros in my number will become ones, any ones will become zeros. Turns out this works a little better with the arithmetic, but it's still not perfect. We still have to have a lot of extra hardware to make this kind of arithmetic work.