The digital world runs on binary. But while numbers made up of ones and zeroes is easy enough to get your head round, what happens when you need to express a negative number in digital form?

In fact, it's not too dissimilar to the way we put a minus sign at the left-most side of a number: you simply place a sign bit at the left, which takes a value of 0 or 1 if the number is positive or negative. But, as Professor Brailsford explains in this video, it doesn't always go to plan—and it can be embarrassing, or even dangerous, when it goes wrong. [Computerphile]

## DISCUSSION

Umm, most computers use something called 2's complement, which is decidedly more than just a sign bit. And usually works pretty good. Except when it doesn't, of course.