Tax advisers have no difficulty in understanding the difference between avoidance and evasion: one is legal, the other is not. The flippant response that the difference is 'the thickness of a prison wall' is sometimes used to explain this to a lay audience, but confusion is widespread. MPs, NGOs and the media refer to 'tax dodging' and say that companies should pay their 'fair share' of tax, without being explicit about what that means.