Abstract. This paper concerns the semantic difference between strong and weak necessity modals. First we identify a number of explananda: their wellknown intuitive difference in strength between 'must' and 'ought' as well as differences in connections to probabilistic considerations and acts of requiring and recommending. Here we argue that important extant analyses of the semantic differences, though tailored to account for some of these aspects, fail to account for all. We proceed to suggest that the difference between 'ought' and 'must' lies in how they relate to scalar and binary standards. Briefly put, must(φ) says that among the relevant alternatives, φ is selected by the relevant binary standard, whereas ought(φ) says that among the relevant alternatives, φ is selected by the relevant scale. Given independently plausible assumptions about how standards are provided by context, this explains the relevant differences discussed.