Having learnt most of what I know prior to 1998, I've always taught that a kilobyte is 1024 bytes, with a megabyte being 1024 x 1024 bytes, etc. I was just looking at the Edexcel GCSE Computer Science specimen paper, and there was a question about calculating the number of bytes in a Gigabyte, which suggests that Edexcel agrees with me. According to the Wikipedia article on the kilobyte, however, the IEC defines the kilobyte as 1000 bytes, and 1024 bytes is a kibibyte, which I'd never heard of. Apparently this was agreed in 1998. What do you tell your students? Do you mention that it's an area of contention?