At the heart of most assessments lies a set of questions, and those who write them must achieve "two" things. Not only must they ensure that each question elicits the kind of performance that shows how "good" pupils are at the subject, but they must also ensure that each mark scheme gives more marks to those who are "better" at it. We outline a general taxonomy of mark schemes applicable across different subjects and question types, with the aim of improving the quality of marking in many different national contexts. We begin by arguing for the acceptance by all concerned of an "Importance Statement" which expresses what matters most in teaching, learning and assessing a particular subject. Based on extensive studies of exam papers in the UK, and utilising the experience of many examiners, we have developed three specific taxonomies for the kinds of mark scheme that are used with different kinds of questions. The taxonomies show how existing mark schemes vary in their effectiveness, and how they can be improved so that better performances get more marks. (Contains 7 figures.)